It doesn't have anything to do with Epic, it's because Steam provides a great service with a ton of features nobody else offers, and Valve has demonstrated time and time again that they make policies that benefit consumers.

It would be great if Steam had some competition, but Epic ain't it. What people want is another service of equal quality to Steam. Instead the best we have is GOG and that still falls well short of feature parity nevermind the anti-consumer cesspool of Epic.

Suing Valve isn't going to do anything to improve the situation. Realistically what could Valve do to be "less of a monopoly"? Lower the percentage they take of sales? Consumers wouldn't see any benefit from that only developers. Ironically it would also increase Valves monopoly because if they took a smaller cut there would be even less reason for companies to sell on Epic as Epics lower cut is literally the only reason developers (outside of Epic literally paying some of them mounds of cash by way of exclusivity contracts) pick Epic over Steam.

If Epic really wants to do something about Valves monopoly it's simple, they just need to offer all the same features that Steam does. Things like family sharing, streaming support, a cross platform store and launcher, and an excellent review system so people can better understand the games they're thinking about buying. Until that happens yes people will stick with Steam because it's the objectively superior experience.

It isn't a monopoly because they don't require you to use their store. Epic has a monopoly of epic exclusive games.