it's not. choosing to buy on steam because it's a better experience to you than egs is exactly the result we want from competition. they competed for your favor, steam won, and egs lost. personally, sometimes i buy on gog because i like its features better, especially the offline installer and lack of drm, but even if steam won all rounds that would still be competition, they're just good at it.

anti-competitive measures are the ones that try to abuse an existing market position to take that choice away from you and force you to go one way or another. if you really wanted a grasp on valve, you could argue for example that the steamdeck is anti-competitive on the market of game stores, because it makes using competing game stores inconvenient (even though you absolutely can do it, i have played uplay games on my steamdeck, and could probably easily install egs as well, i just don't have any reason to try). exclusivity is also a very clear-cut anti-competitive measure, because it just cleanly takes choice away from the end user and forces them to go with a specific launcher, or worse, specific hardware in some cases. but just being better than everyone, or as a consumer choosing to go with the best option is not anti-competitive, it's just winning the competition