Steam only hosts the code incidentally. It manages your licenses. This is a big difference, and it's one publishers are free to work around if they wanted to.
When it comes to DRM and content hosting, the recent closure of the Microsoft ebook store comes to mind.
If a marketplace is underutilized it risks becoming obsolete. And, unfortunately, sometimes that means loosing access to content.
There are additional considerations, of course. Steam is also a social network of sorts, so fracturing gaming communities is also of concern. But it could also just be that gamers are accustomed to Steam; they like it. And they don't like platforms that don't play well with it.
Are you saying that Steam/Valve doesn't have any lock-in contracts? So, if my game is sold on Steam then I could let people 'port' it to another provider?
If by "port" you mean remove it from a steam accounts library and add it somewhere else on a different platform then no, I never heard about something like that. But you are free to give an extra copy to the other account, while accepting that some people will abuse this. Just like you can let people buy the game somewhere else and give them steam keys that can afaik be generated for free while missing out on some steam features like reviews being counted.
Or do you exclusivity contracts? There are certainly games that were sold on Steam and are now exclusively available on Epic's Store. Satisfactory is an example that comes to mind. Copies sold on steam are still honored there, so the contract probably doesn't allow you to force users off steam after the sale.
In general I'm quite amazed at how little abusive practices Valve has shown while having a quasi monopoly for such a long time. Stagnation is probably the one, if you want to count that. I hope they won't become worse under pressure. And I certainly have little trust in other companies trying to replace Steam to behave as well.
GOG Connect may be along the lines of what you're describing. The compatibly of games is on a temporary basis set by the publisher, but, yes, it does allow you to activate Steam purchases on GOG.
This is somewhat myopic -- there would be no incentive politically or monetarily for a publisher to a) track these licenses to the individual or b) give out new licenses to previous purchases.
If a marketplace is underutilized it risks becoming obsolete. And, unfortunately, sometimes that means loosing access to content.
There are additional considerations, of course. Steam is also a social network of sorts, so fracturing gaming communities is also of concern. But it could also just be that gamers are accustomed to Steam; they like it. And they don't like platforms that don't play well with it.