Not parent poster, but I'm going to see if I can come up with some.

0: If you get banned from Steam, you lose hundreds or thousands of games.

0.1: You can't use credit card chargeback protection since you will get your account banned.

0.5: If you're blocked by VAC anti-cheat, you're locked out of all your games that use VAC.

1: Steam requiring other storefronts to sell at the same gross price instead of the same price net fees. This means nobody can compete with their 30% cut... On the other hand, they take 0% for activating games sold elsewhere, which kinda balances it. Still, this is probably the biggest barrier that's maintaining their 30% cut.

2: Discoverability since they stopped curating the games list. (Maybe? Not sure if this is a problem, tbh.)

3: Normalizing the concept of games requiring a launcher to run/DRM.

4: Offline play functionality is inconsistent, so sometimes it breaks when people are traveling with no Internet access.

5: Porn games can be seen easily my minors/people who find it offensive.

6: Region-locked censorship, like gore in Germany.

7: Some people would say region-adjusted pricing, but I disagree. Still, might be a valid reason for some.

(Numbering is wonky because I thought of actual real problems later.)

I think I did pretty well! It's hard to find things to fault. It's a pretty great platform.