They didn't get one punched because he doesn't always go straight for the kill, nor does he even use his serious moves most of the time.
For example:
Saitama destroys Boros' armour with a punch. Why didn't Boros die? Well neither did Hammerhead, whose armour Saitama also broke without killing the person inside.
Then Saitama blows Boros' arm off. Again, clearly didn't go for the kill.
Next he serious punches Boros, but Boros has a pearl inside that lets him regenarate unless you also destroy it.
Next punch is the serious punch that kills him.
And as I keep telling YOU, Saitama's point of existance is to be able to beat Everyone/everything/everywhere.
Because punch>everything
Actually, Saitama can be defeated, at least that's how I think. We know that Saitama's strength grows exponentially when he's in his serious mode we saw that strength graph during his fight with Cosmic Garou. However, it seems that this only happens when he's in his serious mode.
Therefore, Saitama, without being in his serious mode, has a power limit (his power is fixed). If the person he's facing is far above this limit, Saitama could be defeated before he has the chance to enter his serious mode and evolve every second or minute.
But, to be honest, as a Saitama fanboy, I don't want that to happen.
Cosmic Garou could copy Saitama's strength, but seconds later, Saitama became even more powerful, and Cosmic Garou couldn't keep up with his rapid progression. So, the only one who could defeat Saitama was the future version of Saitama, if you catch my drift. LOL!