The competition between USA, Russia, and China is what gets people to tune in, really. With anyone of those countries not participating, viewership tanks, is my guess.
There’s also the whole thing about eschewing politics. Traditionally wars were paused for the Olympics, but I’m pretty sure that’s irrelevant today, especially when the US led a boycott of the Moscow Olympics in 1980 and then Russia boycotted the Los Angeles Olympics in 1984. It’s really about viewership.
Oh and the IOC is notoriously corrupt. If Russia pays to participate, the IOC will find every which way to let them, no matter how badly they cheat and genocide, same with the US and China.
Like I said, it’s about viewership, and the UK brings very little of that compared to the US, Russia, and China.
If you really cared to see the best, you’d know who Simone Biles is. Everyone tunes in to see their guys win.