They could have easily crammed the Steam Deck full of stuff to make it hard to use for piracy - locking down everything, making it usable only to play games you legitimately own, force you to go through who knows what hoops in order to play games on it. That’s what Nintendo or Apple or most other companies do.
But they didn’t, because they realized they didn’t have to. It’s 100% possible to put pirated games on the Steam Deck - in fact, it’s as easy as it could reasonably be. You copy it over, you wire it up to Steam, if it’s a non-Linux game you set it up with Proton or whatever else you want to use to run it, bam. You can now run it in Steam just as easily as a normal Steam game (usually.) If you want something similar to cloud saves you can even set up SyncThing for that.
But all of that is a lot of work, and after all that you still don’t have automatic updates, and some games won’t run this way for one reason or another even though they’ll run if you own them (usually, I assume, because of Steam Deck specific tweaks or install stuff that are only used when you’re running them on the Deck via the normal method.) Some of this you can work around but it’s even more hoops.
Whereas if you own a game it’s just push a button and play. They made legitimately owning a game more convenient than piracy, and they did it without relying on DRM or anything that restricts or annoys legitimate users at all - even if a game has a DRM-free GOG version, owning it on Steam will still make it easier to play on the Steam Deck.
Not to defend the shitty companies out there, but in a sense they have no choice. Once you’re publicly traded shareholders expect infinite growth at ANY cost to the consumers or the employees of the company. Every single year they expect to see their return increase, even looking like a plateau for a short period is enough to make a huge chunk of the greedy bastards jump ship. IMO shareholders are the number one, most direct and largest cause of the enshitification of everything. Being publicly traded these days is a death sentence for a companies nature and good will.
And its our fault too. Its easy to see shareholders as rich fatcats telling the CEO to “Put MTX in it and make it slow and grindy!”, but if any of us have IRAs or retirement accounts, we are the shareholders too. We want the nest egg we set aside to grow, and that leads to the same problem.