There are a couple things that cement Steam at the top, imo.

The first is obviously familiarity and completeness - everyone knows Steam and it almost certainly has whatever game you are looking to buy.

The second is added value - very good controller support, mod support, (some) good community spaces, easy multiplayer sessions, proton integration, regular updates, etc. These are genuinely worth using the launcher for.

The third is the single-launcher desire (or the sunk cost effect depending on your point of view). People already have massive libraries on steam, keeping their collection there just makes it easier to browse and find whatever they want to play. Yes, there are other solutions for this, but that is an extra step - there will always be people who don't take that step.

Familiarity can't exactly be bought - Epic tried. Completeness can be bought (Epic, Gamepass) but it is hideously expensive. Added value is difficult to get to parity (mod authors aren't going to instantly port to a new launcher even if there is support). The single launcher effect is a massive first-mover advantage.

I completely agree that Valve having a monopoly on PC game distribution is unhealthy long-term, they also just offer a better experience to me as a consumer than the competition. Even when games have GoG versions, for instance, they are often not updated as frequently and mod support is often more difficult, controller support is hit-or-miss, etc.