You aren't allowed to bring your own soccer ball, basketball, football, etc. Why do we allow some items that have a huge impact, to be up to the player/team?
On the other hand, there are many championships where all drivers must drive the exact same car, or the cars are heavily regulated in terms of power output. Here it's all about the raw skill of the driver.
There is room for both of these in running.
Until recently, swimming advances didn't matter all that much. It's only in the last decade or so that the fabrics became better than the swimmer's skin, and they started covering up more of the body. That gave substantial advantages to swimmers with a ton of money.
(Substantial being relative, of course. We're talking 1% improvements, so you're not taking on Michael Phelps in one of these, even if he were wearing a ball gown. But it's the difference between gold and not qualifying for the final heat.)
Cycling similarly allows picking the bicycle frame to match the rider’s body. It does have a minimum weight for bicycles, though (6.8kg, which is high for current tech. Given deep enough pockets, 2.7kg was possible in 2010. See https://www.tririg.com/articles.php?id=1035_Worlds_Lightest_...), and regulations that effectively forbid recumbent bikes.
In some cases, flexibility also is there for historical reasons. In soccer, not even field size is fixed (with exceptions for top levels) for that reason (fields inside athletics tracks tend to be longer and less wide than fields inside stadiums specifically built for soccer)
A soccer field must be rectangular, but can be anywhere from 45 to 90 meters wide and anywhere from 90 to 120 meters long (https://resources.fifa.com/image/upload/spielregeln-2019-20....)
So, minimum size is 90m × 45m, maximum size is 120m × 90m. Area-wise, that’s a factor of 2⅔.
Source: https://operations.nfl.com/gameday/pre-game/nfl-game-ball-pr...
The list can basically go on forever. With no real "a priori" idea on where you have to stop to no longer find a benefit in equipment. Could even go back to "access to gym" or "access to trainer" and expect a level of influence.
For example, athletes from wealthy countries are exposed to better training techniques and apparatus. Is that fair?
That said, the equipment sponsorships are a major part of the equation, very expensive, and very competitive. I think it forms a larger part of the budget for less successful teams, where title sponsorship is less lucrative. And in any case, it's its own form of "title" sponsorship.. I'm not sure how much benefit Pinarello would get from being on the jerseys - everyone knows Ineos rides Pinarello.
But anyway, I think my point that this lucrative competition prevents standardization is valid.
https://cyclingtips.com/2017/05/200-bikes-and-a-lot-of-cash-...