Because he's scared. Zenitsu is an incredibly powerful
Through sheer will and anger, Zenitsu was able to overcome his cowardice and stay awake in battle against demons, shown during his bout with Kaigaku and Muzan, especially in the fight with the latter in which he refused to give up and persisted despite being enervated and injured.
So, to sum it up, his fear of going against terrifying demons that could end his life, drives him to fall asleep. However, in an unconscious state, he becomes a totally different individual, who takes mere seconds to slay a demon and get the job done.
Zenitsu does, however, appear to retain a certain degree of awareness while asleep. Even during Demon Slayer's "Mugen Train" and "Entertainment District" arcs, he continued fighting in his sleeping state, rather than just unleashing a single blow as he had at the start of the series.
But Zenitsu is not just powerful in his unconscious state. His real power shows when he is aware and awake, he was able to outmatch the new upper rank six demon, Kaigaku. At this point Zenitsu has manifested his speed and power, and he was also able to keep up with the Demon King, Muzan, even when weakened.
Shinobu Kocho is a capable Demon Slayer whose worthy of being the Insect Hashira. That being said, she's arguably the weakest Hashira when it comes to pure strength.
He doesn't realize how strong he is because his anxiety holds him back. While unconscious, he can have a clear head and fight back, giving Zenitsu an edge over his fellow slayers.
Zenitsu mastered the first form to its full capacity but failed to learn the rest. However, the first form itself is extremely difficult. With his master gone, Zenitsu can no longer learn the remaining five forms. Therefore, during his fight with Kaigaku, an Upper Moon Six, he created a 7th form.
why tanjiro's head is so hard? Well, it's thanks to his genetics, as his mother, Ki Kamado, passed down the trait of a rock hard head to him. In fact, Nezuko heard a story about their mother. scaring away a boar by headbutting it, which shows that Ki was also very tough and fearless.
Exploring Zenitsu's fighting abilities in Demon Slayer
But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on. Zenitsu has perfected the first form only and is seen performing it only when he's unconscious.
Unfortunately, the answer to this question would be, no, he did not. Zenitsu, along with Tanjiro and Inosuke got way stronger than average corps members with training. In fact, he became one of the very few to even invent a new form of Thunder Breathing. However, his dream of becoming a Hashira was never realized.
The reason for this is pretty simple. Zenitsu wasn't able to master the other 5 forms. He never wanted to learn any breathing techniques. or fighting techniques, nor he wanted to join the Demon Slayer Corps. He was forced, not in a bad way, to train because his master, Jigger Okuwajima, saw potential in him.
Even if this only happens when he passes out, Zenitsu clearly still has the power to help himself and others. One major part of why Zenitsu is scared of demons has to do with his Breathing Style Form mastery. Zenitsu uses the Thunder Breathing Style, but has only mastered the First Form for combat use.
Who is the most stronger between they two? Zenitsu is definitely faster but Tanjirou is more tactical and more versatile. Tanjirou could lose if he has no idea what Zenitsu can do, but if he does he can easily anticipate. Currently in the manga, Tanjirou is definitely stronger.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers.
However, the scar changed its appearance during the Final Selection when the Hand Demon gave Tanjiro a major blow to the head. The blow not only increased the size of his scar but changed its colour as well. The scar changed even more when Tanjiro fought against Gyutaro and Daki during the Entertainment District arc.
Tanjiro wasn't born with this scar on his forehead. Rather, he acquired it when tackling his younger brother out of the way of a falling pot of boiling tea. At the beginning of Demon Slayer, the scar was a light-pink, circular mark.
During his life, Tanjuro was stricken by an unnamed disease that caused his body to be so skinny and frail, and his skin also appeared quite pale. In his later years, Tanjuro spent most of his days bedridden.
This mastery allowed Zenitsu to form a Seventh Form of Thunder Breathing that is unique only to him: Honoikazuchi no Kami. While this move might seem similar to Thunder Breathing's First Form, the speed and power of this technique are on a whole new level.
5 Thunder Breathing First Form
Despite this, Zenitsu has completely mastered the first form and even created more powerful variants of the technique, turning it into one of his most potent attacks. With the thunder-breathing style's first form alone, Zenitsu has managed to overpower and decapitate many demons.
Thunder Breathing has 6 standard forms in total, with the first form said to be the foundation for all the other forms. However, Zenitsu Agatsuma developed a Seventh Form in order to stand on equal ground with Kaigaku, his former senior disciple under Jigoro who had become the Upper Rank Six of the Twelve Kizuki.
After mastering his Thunder Breathing, Zenitsu would eventually become capable of awakening his Demon Slayer Mark. As this mark would enhance his strength and speed, he can overwhelm Gyomei to a certain extent, but he still won't manage to defeat him.
Zenitsu has some combination of RBD, narcolepsy, sleepwalking, and lucid dreaming that allows him to behave as if he were awake while asleep. Thanks to his extensive training and superhuman hearing, he's able to detect and fight demons in this state.
Zenitsu Gets Reincarnated As Nezuko's Brother.