Board Thread:Questions and Answers/@comment-31724751-20170529101655/@comment-25322628-20170605005242

Except Goku and his kind have something called a Zenki or something that enables them to increase in power only after recovering from a near-fatal injury, which implies that Goku, only after surviving near fatalities and having time to recover from them, has a limit to how strong he can become.

Saitama has no such ability and may just hardened his bones and internal organs to prevent damage.