It's diminishing returns.
We need a giant leap forward to show a noticeable effect now.
Like, if a cars top speed was 10mph, a 5 mph increase is fucking huge.
But getting a supercar to top off at 255 instead of 250, just isn't a huge deal. And you wouldn't notice unless you were testing it.
So even if they keep increasing power at a steady rate, the end user is going to notice it less and less everytime.