Made by Nintendo means that it'll be a super locked down device that only plays games made by Nintendo or a rather small list of 3rd party game makers. Developing for the platform is expensive and requires an extremely lengthy certification process. This means that all the games are reasonably high quality, sure but it also means that small developers or games with some adult content will never make it.
The Steam Deck, on the other hand runs an enormous library of Steam games and new games crop up every day. It also runs Switch 1 games! The barrier to entry is tiny and it's actually possible to mod games which is probably the single most important feature in modern gaming if you want your game to last and be popular for a very long time.
The Steam Deck also runs Linux which means hackers all over the world can make it better. Even simple shell scripts that automate common tasks provide an enormous benefit! You can automate synchronizing your save games between your PC and your Steam Deck wirelessly, for example without much effort because it's just (mostly) normal Linux.
The Steam Deck is general purpose hardware in a portable form factor running a general purpose operating system that's been optimized for (portable) gaming. If you want a feature you can make it happen yourself or ask the monstrously huge (and obsessed) Linux community for assistance.
The Switch is locked-down, application-specific hardware in a portable form factor running an application-specific operating system that's severely locked down and can't be modified or improved in any way by end users. If you want a feature you have to ask Nintendo and pray.