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.
I think you meant to say "Deck" in the second paragraph.
But yeah I totally vibe with your observation. Something a bit ironic with this situation is that a big part of why other companies simply can't provide the kind of service Steam does is copyright issues - XBox and Playstation both give out free games, Nintendo has their online service, but no option remotely compares to "make everything available on one app on the most modern device." Imagine if Nintendo put everything that had ever appeared on the Wii/DS/Wii U/3DS/Switch shops all on one online storefront on the Switch, and let you attach ownership to your account and play everything you owned on the most recent device - then they would have about a quarter of the functionality that Steam has on the Deck, where you have access to every game you've bought for PC for as long as Steam has existed (and quite a few things from before that) and the number of things that have lost compatibility is pretty low.