We say this because when you keep doing pushups for 50+ repetitions, at a certain point, your muscles are really just adapting to this particular movement, and this particular weight, rather than being stimulated to grow. So someone who is able to do a huge number of pushups, might fail miserably if you asked them to do a few with say 10kg of added resistance.
It's not like there are no benefits though. While the strength gains are limited, improvements in stamina is the takeaway.
If you look at the bodies of seasoned marathon runners, you'll find that their muscles aren't particularly large. But they are superbly durable over time, and can keep going for hours on end without pause.
Then compare this to the body of a 100m dash sprinter, whose body is meant to perform explosively over short periods of time.
And then to a weightlifter, who is, instead of aiming for long periods of movement, aims for one single rep.
If you compare the bodies of these different types of athletes, you'll find how the difference is easily spotted with the naked eye.
This is the reason why our calves don't grow to the size of a volleyball when we climb mountains. They gain the strength needed to carry our bodies, but after that, they gain the stamina to keep doing it, rather than gaining the strength to carry ten humans.
Quantifiable differences though, isn't something I'm equipped to answer. This is more aimed at the question in the title.