Design is harder than you think. Fan-made character sheets are not better than professional ones; it's not an easy thing
I would disagree with the second part here.
Fan-made character sheets, are, in my experience, often significantly better than professional ones (thought also sometimes worse, or flawed in exactly the same way).
The issue I frequently see with professional sheets is that basically they've decided to make attractive character sheet, that contains all the information they
believe is needed to play the character, but they tend to be shockingly lacking in practical terms.
In particular I'd identify the following common faults:
1) Information that is referenced infrequently is given parity to information that is referenced infrequently, or is much more obvious, visually. This can get really bad - some piece of information that, in practice, comes up every session multiple times, is often in small text, sometimes not even on the first page.
2) Completely unrealistic and clearly untested amounts of space for things. This is a striking flaw. I can't even count the number of official character sheets where will have like, 3 lines for weapons, when the average PC will have like 4-6 (often including example characters), or take up 1/4 of the entire front page with room for some sort of write-in bit, where most PCs will only fill in maybe 1/4 of that space. Or vice-versa - you'll have a situation where a normal PC has a 1/2 a page of special abilities, even written small, and they've given you less than a 1/4 of a page.
3) Simply missing information. How does this even happen? I've seen countless official characters that just don't have a space for stuff you need to know to play the character or that the book strongly encourages you to write down. Derived stats can sometimes be particularly bad here, because they're particularly likely to be just be missing.
4) Visual design that's clearly antithetical to way the sheet is used. This is a bit different from 1, though I guess related, in that it's typically less about text or positioning, but more often about bad use of textures/pictures/boxes/lines that make it harder to use the character sheet.
You're right in that some fan-made sheets have all these flaws and more. But many fan-made sheets, in my experience, are the product of actual play, and actual play shows you what matters and what doesn't, and what can afford to be small and take a minute to reference, and what's actually pretty vital to have immediately.
And frankly, many official character sheets are so bad that it's impossible to believe that they're the product of playtesting. Rather it seems like they're the product of theoretical thinking and or just cramming information in (sometimes not even in an organised way).
There's also wild variance. Character sheets haven't really improved much, unlike most aspects of RPGs. Rules now are typically presented far more clearly and in a far more organised way than in, say, 1995. Mechanics make more sense and are more playtested, and mathed-out, instead of guesswork and "feels okay" numbers. Rules are typically carefully considered in terms of how they impact the game, what behaviours they encourage (despite the fact that people actively opposed this to some extent in 1995).
But character sheets? Pfffft. Your odds of any given game having a good or even decent character sheet now are about the same as they were in 1995. The only real place that has improved here is PtbA games and the like, where your whole class/character is one sheet, because those do mostly seem like the product of playtesting of a serious nature.
If you see something done badly, then you have found a great (and possibly lucrative!) opportunity to do something better yourself.
Often, when making the better version yourself, you realize why all the other ones are done badly, and that it is not that easy.
This is true. But there's also a significant portion of the time when you're left going "What, why? Why did they do such a terrible job?". Of all the character sheets I've designed for games (most lost to the ages and old HDDs sadly now), the only one that made go "Damn this is hard" is 2E AD&D. But I was able to design a sheet that worked better than the ones we had, and as a bonus made people think about their character a bit more and numbers a bit less.