Well, I'll try to explain why. (But buckle up, I'm a scientist, engineer, and a statistics nerd in real life. I can go on about this stuff for days...I'm often paid to do exactly that!)
First of all, I agree with you: he
could have lots of things, game-wise. But we don't know any of that for certain. Sure, we can make assumptions and educated guesses, but even the best assumptions introduce a margin of error...possibly even compounding error, if we start making assumptions that rely on other assumptions. We assume proficiency, then assume a class level which modifies that proficiency bonus, then assume enough class levels to also assume feats that also adjust other things...we are multiplying error upon error. This isn't helpful, because the more guesswork we use, the more wrong we are likely to be.*
So for the lowest margin of error, we would use the rule that requires us to make the fewest assumptions for missing data--not what we "can safely assume," not stuff that "feels right," but
data...things that have been measured and recorded.*
The only data we have is the distance he jumped. We have very good data, to: carefully-measured, verified, and reviewed data. We know that he jumped 8.95 meters, and that's really the only number we have here in the real world that is also in the Player's Handbook. All else would be conjecture and assumption, and therefore unreliable...and that's the long-winded answer to your question.*
Again, I do agree with you: he
could have lots of things, game-wise. And if he were a D&D character, he certainly would. (Well, unless he's an NPC, in which case he might not have levels. And we don't know if our DM lets us use Feats and Multiclassing either, or...) But Mike Powell is not a D&D character, he's a human athlete...and a very impressive one! I'm not trying to create a Mike Powell 5E character sheet; I'm trying to picture what the D&D rules look like in real-life, and how incredible these athletes look through that lens. Imagine: there are humans walking among us who are stronger than the giants and dragons of D&D!
*Yes, this is a stretch. These are rules for a tabletop roleplaying game, not the Second Law of Thermodynamics. All of D&D 's rules are going to look absolutely absurd when compared to real life, and one moogle's absurdity is another's doctrine. It is laughable to think that the rules for Long Jump are somehow 'closer to real life' than the rules for a multiclassed Fighter/Monk. All I'm asking is that folks laugh with me here: you asked me why I wanted to use one ridiculous equation instead of a different equally-ridiculous equation, and I'm trying to give a serious and thoughtful answer.
I don't mind, except that after a while it starts to feel like I'm being credited (blamed?) for what is written in the rules. I didn't create the rules for Long Jump, and I'm not adding or removing anything to them. Clearly the equation is problematic, but it's not my equation to fix.