I’ve updated my original post with a testimony proving my point. For this case, I can only guess that it is the same case as with Mindustry: Because they have not been made aware that they are getting a worse deal.

I’ve updated the original comment with a testimony about the topic which also contains an answer for your question:

In the rare occurrences where we’re made aware that the game is being sold on another platform for a lower price, …

I guess no one has made them aware yet.

Cause yes valve does prevent you from selling your game on other platforms at a cheaper rate, so long as you are doing so via steam key or when valve servers will be the source of distribution.

This is false. Even if I put the game up on my own server, letting you download a zip directly from there, cutting valve fully out of the picture service-wise too, I am still not allowed to sell it cheaper on my site then on steam.

Edit: Based on the score of my comment it looks like I need to provide some more evidence to back this up, I thought this was already common knowledge. https://drive.google.com/file/d/1bwajZMNAof74mSNRMcTAVlF8m0fplHkA/view This is just an excerpt of the full testimony, I don’t know if the full file or an unredacted version exists anywhere else, feel free to share if you found one.

The first part is about steam keys, but it starts to get more interesting from page 16 of the pdf onward, so feel free to skip to there. TLDR: According to the testimony, valve does not actively check for price parity, but if they become aware that a game is sold cheaper elsewhere, they contact the publisher asking for the reason for the price difference and may threaten to reduce promotion of the game on steam or straight up remove if from steam if they do not match the price.

I have not found any of the E-Mails that were mentioned during the testimony, but it seems this happened more than just a few times for something “That is not our typical process.”.