Regular pushups cease being a strength-training exercise pretty quickly. You're being ridiculous. Once you're doing 40 in a set it's well into endurance territory, and you're not making muscles any bigger. It's like going for a run every morning, and slowly increasing distance as you progress, but for your upper body and in a lot less time commitment. We're not increasing the weight, probably decreasing it actually.

It's just at the start you're doing strength-training if you've never done upper body exercises, because your upper body is atrophied.

Also note I said ~maxreps, the tilde was intentional, it was meant to communicate "around your maximum" to express the importance of increasing it as your endurance improves.