Mega evolutions are just like shots of genetic steroids that cause your Pokemon to go into an evolutionary roid rage until they wear off at the end of the battle. It's a forced, short-term evolution, more than anything.
They are evolutions it's even in their name, the only reason are like this is because they are mostly evolutions for pokemon that can't evolve naturally which are most 3 stage pokemon as well as legendaries with some 1 stages thrown in.
Basically they are evolutions for those that can't have evolutions, and are temporary both as a balance and not to take away from the fact that his pokemon can't natural reach any higher state than they are already at.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me