Only because there's no means to. If chess pieces made out of different materials made you better at playing, the best players would use the most expensive pieces. The Free Market means people who can pay more for an advantage will pay more, and there'll be those who satisfy that demand, and those who see the opportunity to exploit/create such opportunities as in the case of pay-to-win DLC. Just like in-game 'consumables' with a fictional shortage created just for the purpose of generating on-going revenue. These didn't exist when there was no mechanism to supply them, but the invention of a means to charge for such a supply shortage meant it was created and exploited.
Then the game would no longer be chess. It would be a modified version that would almost always be won by the richest player and be quite boring to observe. In the age of e-sports, I can see why keeping the playerbase as large as possible could be perceived as beneficial, and why framerate would be focused on due to its effect on reaction times.
You literally cannot stop 'professional' advantages being offered to gamers, and it'd be financial daft for any company to offer some egalitarian ecosystem unless they could leverage that to generate more sales and revenue. Well, that's really not possible with mid-gen upgrades where the higher tier is going to get better resolution and/or framerate, both being advantageous in competitive gaming. Capping online modes to the lowest common denominator would cripple appeal.
You literally can stop some professional advantages as PS4Pro guidelines illustrate.
I agree that it would be financially daft to offer an egalitarian ecosystem that can't be financially leveraged, but it's also true that it would be financially daft to leave a portion of the playerbase behind when simple stipulations to developers can prevent that.
I keep coming back to wondering how much it really matters. I genuinely don't know as I'm not really one for multiplayer games - if I play them, I play them for fun.
So there's that: how much difference does framerate make in a competitive sense?
There's also the matter of whether it should be up to the platform holder or the developer/publisher? I generally think that it should be the latter, even if limiting the framerate across the ecosystem would be wise.
The financial counterpoint to that is that you never know which competitive multiplayer game is going to explode in popularity and, by limiting factors which are deemed necessary to have a level enough playing field that it makes for engaging spectatorship, you have a much larger playerbase from which you may draw. Then you have more competition, more spectators and the cycle feeds itself.
Dunno. My gut tells me that Sony are taking the wiser approach, but that depends on the difference that framerate makes.