It's one thing for Google to have a problem with something and quite another to anti-competitively leverage their other market positions as the solution.
Other comments describe a few specific examples of that abuse (like also thwarting Epic's attempts to be preloaded by OEMs), but I think it's more interesting to look at how money is changing hands as evidence of market power being abused.
In particular, consider that Google in theory should be fine accepting payments as low as their hosting costs (and probably much less since their app store helps attract phone purchasers to fuel their ad network, since they also take a cut from OEMs, etc). Compare that to each individual developer on the store who in theory is able to afford at most some large fraction of revenue for the privilege of being on the store. Note that the latter number is (usually) much greater than the former and that the latter is what's actually being paid. If there were any real competition for something as trivial to build as an app store then you wouldn't expect Google to be able to successfully price discriminate across so many orders of magnitude of prices and extract nearly all of the surplus value from the end user purchasing an app. The fact that they're able to do so and are doing so should serve by itself as a strong indication that something suspiciously anti-competitive is happening. My $20 app wasn't 20x harder for Google to host than your $1 app, so why are my fees 20x higher? Because they can get away with charging that much because they stomp on any competition.
That isn't any kind of nail-in-the-coffin proof mind you, but it serves as a useful heuristic for spotting abuses of market power in many markets. Throwing that same idea at the right to repair movement, the amount paid to John Deere for tractor repairs is many times greater than what people have been able to achieve on their own (when they've been able to do so), and the only barrier is DRM that was added to prevent unauthorized repairs. If John Deere were really that much better at repairing their tractors than third parties then they wouldn't have to artificially exclude everyone else from attempting it to retain market share and command those prices.
Nobody's saying that Google or John Deere shouldn't be allowed to profit from the things they make, but using existing power to hamper competition and seek greater rents isn't good for society as a whole.