My reasons are as follows:

• The feature parity with Steam’s social functionality is bad. There’s no Messaging, no Friends list with the ability to join their games, No chat, no stats page, no achievements, no screenshots, no streaming, no trading cards, nothing to make the experience interesting outside of buying games.
• The IAP experience is not there, and if it is, they are not holding vendors to it. When I played Zenless Zone Zero, I was quickly funneled to Hoyoverse’s launcher for all of that. Its incredible Epic is letting the 1-2% they could make here slip from them, and it means Hoyo needs my Paypal or whatever (i’m not comfortable with this).
• Developer API support is not there, which means that any additional EGS integration is, well, half baked. See Satisfactory for an example
• Steam offers better game discovery, family sharing, matchmaking, and so forth. EGS provides none outside of what it plans campaigns for.
• Steam provides a better API for game controllers, EGS assumes the game will use XInput which works on a per-game basis. Its nothing compared to how with Steam, I can remap controllers easily, especially on KBM only games like Factorio.
• Epic Games acquisitions of exclusives means they have little financial runway, and projects that I liked (Alan Wake 2, World of Goo 2) will probably never see the light of day outside EGS. This means the developers took a constantly shrinking advance over genuine sales and WOM, many are still not profitable. Its already unsustainable for Remedy: They’ve killed the deal with Epic and are now going to a shaky Annapurna. Gearbox didn’t profit off Borderlands 3 off its advance, and they aren’t getting any further profits from the shrinking advances.

Most importantly: EGS is supported by Fortnite money from parents who don’t know any better. If Fortnite collapses (which can happen easily if regulators hold Epic to task), EGS will keel over.