It's a mystery that's gradually revealed throughout the manga. But basically, it's due to his intense training and determination.
Saitama's strength in the manga comes from his intense training and unwavering determination. He pushed himself to the limit every day, which eventually led to his overwhelming power.
Saitama obtained his power through a rather unconventional way. He simply trained non-stop, doing mundane exercises like push-ups, sit-ups, and running. This extreme dedication transformed him into the powerful being he is now. It's a simple yet unique origin story.
The Flash got his powers through a freak accident involving chemicals and a lightning strike. It's a classic origin story in the comics.
Deadpool got his powers through a messed-up experiment. His body was subjected to a procedure that gave him superhuman abilities, but also left him scarred and crazy.
Kraven gained his powers through a combination of genetic enhancements and intense training in various combat and survival skills.
The Hulk got his powers through a gamma radiation accident. Bruce Banner was exposed to a huge dose of gamma rays, and that's what transformed him into the Hulk when he gets angry.
Morbius gained his powers through a failed experiment aimed at curing his rare blood disease. The process transformed him into a vampire-like being with enhanced strength and other abilities.
Deadpool got his powers through a rogue experiment. A weapon program tried to give him enhanced abilities, but it went awry, resulting in his unique powers and messed-up appearance.
Kraven got his powers through a combination of genetic enhancements and intense training in the wild. He developed enhanced strength, speed, and senses.
Mysterio acquires his powers through a combination of advanced technology and illusion-creating skills in the comics.