Yeah, we only have to look at the FTC's lawsuit against Amazon to see what they consider an antitrust problem:

[...] Amazon violates the law not because it is big, but because it engages in a course of exclusionary conduct that prevents current competitors from growing and new competitors from emerging. By stifling competition on price, product selection, quality, and by preventing its current or future rivals from attracting a critical mass of shoppers and sellers, Amazon ensures that no current or future rival can threaten its dominance.

That isn't what we see from Valve - in fact it's the opposite, as Valve's strategy with Steam is simply to provide the best service and be the gold standard. The competition is almost always compared unfavorably to Steam, because gamers know how it feels to use a mature platform that isn't trying to abuse them.

Valve has even taken some steps that wind up increasing competition in adjacent markets, such as operating systems (Proton has contributed significantly to Linux popularity) and even handheld game devices (Steam Deck set off an arms race when electronics manufacturers realized Nintendo is asleep at the wheel). Steam is as pro-consumer as it gets, with the exception of GOG and possibly itch.