Drug league could have people dying mid competition, as they took too many drugs that they had a heart attack when trying to go all out, and that would create so much backlash and vicarious trauma that it gets shut down hard. I think it's the main reason why it doesn't exist.
I imagine that many of the people who would participate are already taking these things (these are not the people competing professionally today). Perhaps mainstreaming it would incentivize more research into safety and sustainability around artificially pushing humans beyond their current physical limits.
Given the extreme competitive and monetary pressure already involved in sports, as well as expectations of audience, I question how much we can talk about consent and lack of coercion. If a drug league would exist, players would be pressured to take part to the very limit of what's legal, health be damned - as they already are in regular leagues.
Two things:
This is a minority viewpoint. I believe most sports fans and participants want sports to remain free of exogenous performance enhancing drugs.
The second: the situation you describe would disadvantage everyone who does not engage in maximizing their use of such PEDs, and the sport in question would rapidly be transformed from what it is now into something vastly different, as the people willing to ingest these modifications would quickly displace the ones that aren't, or don't embrace the practice as fully.
That may be fine for you, but it is a big change, and many people like it the way it is now.
Personally, I agree with you. There should probably be a drugs league in various sports where people get as insanely artificially enhanced as human bodies can support, I might even watch that despite my general aversion to sports just to see the extreme tech involved. But it's absolutely silly to equate that to what is happening today because "today's athletes are not natural". It's not the same thing at all.