itch.io

Oh, gee, I can't speak for everybody, but:

AAA games don't get published to Itch.io, but they do get published to Steam. There's something about having a small game published alongside big games that makes an indie game on Steam feel more important.

There are many people who exclusively play games on Steam... Valve have actually been sued because of having something close to a monopoly on the games marketplace.

Another reason is that Steam handles a lot of the publishing stuff. That means marketing, refunds, and cloud saves get handled for you.

Valve asks for $100 to keep shovelware off of Steam, as you already know. New games undergo manual review, so the fee helps reduce Valve's workload. You do get the $100 back, though, after your game has sold $1000.

The sad part about publishing on Steam is that they take a 30% cut of sales. This is unfortunately pretty standard as it's also the cut for GOG, Apple's App Store, and the Google Play Store. Epic Games Store takes only a 12% cut, which is second only to Itch.io and self-publishing at 0%.

On GOG: they take 30% but all games published there are DRM-free. For most other platforms, when you pay for a game, you aren't buying that game. What you buy is a license to download and play that game. That license can be revoked whenever the publisher or developers want it to be, making game preservation very difficult. This is not true of DRM-free games.