Hold on. No reason to get snarky, and to be clear, I'll get to your "why is it questions" in a minute. But first, I think when you talk about transfer of learning, you're making an excellent point. BJJ fighters train for BJJ matches. If they want to be able to compete in a Judo tournament, they would train accordingly. If they want to compete in an MMA match, they would train accordingly.
While there is no single bright line from untrained to street fighter, I think it's pretty easy to see the accumulation of skills and complexity. And generally speaking, one's ability to transfer learning from one context A to context B depends on two simple things. How skilled you are in context A? How similar is context A to context B? So, for example, it will be easier for a skilled Judoka to compete in a BJJ tournament than for a skilled TKDist. It will be easier for a skilled Thai Boxer to transition to kickboxing than a Judoka. And so on. A guy like
@Tony Dismukes who has wrestled, boxed, sumo'd, jiu jitsu'd, and ninja'd his way through his martial arts training has a lot of accumulated experience not just in the styles, but in synthesizing the skills and transferring his learning.
MMA is a context in which there is striking and grappling, standing and on the ground. It is pretty much the least restrictive rule set around right now for unarmed fighting, and I would view street fighting (not de-escalation, emotional intelligence, not being a jerk, or any other "self defense" type things) as being on the OTHER side of MMA in terms of skills that may come into play, complexity, scope, and risk. Simply put, if you aren't prepared for an MMA match against someone who is roughly your same skill level, size, and age, how could you believe you are prepared for something riskier, more complex, and potentially broader in scope?
So to your "why is it" questions. The answer is about context. If a boxer gets into a situation that plays to his skill set, he has a better chance of transferring his learning. If a boxer is taken to the ground, whether on a mat, in a cage, or on the street, he's going to be in trouble. If a BJJ guy gets into a situation that plays to his skill set, he's going to have a good chance of transferring his learning. Same thing with TKD, etc.