But they did. When Valve first opened up Steam to non-valve games they signed exclusivity deals with the first games on the system. An example of this is Darwinia, for which digital copies had to be removed from the devs own store and had to be sold via Steam -
https://forums.introversion.co.uk/viewtopic.php?t=40203
Once Steam got traction, I'm sure exclusivity deals were not required because they didn't have any direct competition.
[edit] Re-reading that thread, it's interesting how history repeats itself. 16 years ago, gamers were concerned about dev/storefront cut (it was claimed Steam took 40% back then), lack of Linux support, and general performance of the client
There are certainly some parallels, but given the state of things at the time there was a far stronger emphasis and benefit on the developer's side - you agreed to make Steam your distribution platform of choice (versus hocking keys / downloads via your own website) and give them a cut and in return you got Valve's infrastructure and support behind your game (which at the time was a new and pretty significant offering versus what had been available before - i.e. nothing really).
So I think it's unfair or at least a little disingenuous to say that Valve back then did what Epic are doing now. If Epic
were to do something equivalent it would be more like - look at the current state of things (which granted are different now to when Steam was created), assess the main problems and ways they could improve them, and then launch their own
competing service that does things better (Steam has plenty of issues that a competing service could do better and would feel like a breath of fresh air).
Compete on price, compete on features (for both users and/or developers), compete by directly funding development of new "Netflix Originals" style games to drive attention to their platform**. But instead they are going the Google Stadia or Amazon style route of just blasting $400 million down the pan trying to buy their way in without doing
anything to contribute to the market they are entering. I am not for a second trying to suggest Valve are some messianic organisation that aren't in it for the money too, but they at least appear to recognise that you have to offer
something to justify your place in the market other than bags of Fortnite cash
** And before anyone says it, paying already practically finished game's publishers massive amounts of cash to restrict to their store is
not the same thing as funding development of a title
