Callsignfuzzy makes a good point here.
In addition I would say it's not training that's the problem, rather HOW we train.
My Sensei used to tell us "one year sparring is superior to five years training."
The bottom line is there is no substitute for actual fighting except sparring.
You can be in great shape, fantastic conditioning, speed, but if you haven't put
your body through it's paces against an actual opponent/sparring partner, and
your opponent has, he/she will have the edge on the night.
Seen it happen many times.
They don't. In Reed Kuhn's book "Fightnomics", he ran the win-loss ratios of fighters returning to action and found that, while returning to action sooner was generally better, there were no significant drop-offs in win ratio until they'd been out for about a year. That's hardly "quickly". As well, most of the cases I can think of where fighters stayed out for that length of time, most of the time, the fighters were recovering from some sort of injury, meaning they COULDN'T train for several months, and perhaps some part of their game (footwork for knee injuries, punching power for shoulder injuries, etc) had been effected by that injury.
Because the fighters that most people base that notion off have pushed themselves above their body's natural limits and it doesn't take a huge amount of time off for such a surge in their body to die off.
That's for certain people.
Whenever an mma fighter stays away for jUst a few months too long they go to poop. They are terrible. Why isn't training good enough? There is no practical sense to the matter because it seems learning and training won't make you as good as a guy that fights but if you fight a lot you're going to be so mentally and physically damaged that it's worthless...