You could, but what makes Steam Deck special is that it's SteamOS is built specifically for that hardware, enabling functions you wouldn't normally see in gaming PC hardware.
The difference is that if the device comes with Steam OS, then it's ready to go out of the box and you're assured the hardware has good Linux support.
If it's originally a Windows device then you may have to jump through additional hoops to get everything working. Also you'll have to deal with allowing other OS's in the BIOS if it's locked.
Also you've paid Microsoft for a license you won't use.
The flip side is that there's work to make a native Steam OS build for 3rd party portable devices:
I used the steam deck as a daily driver between laptops. It was good enough to the point that if I had a decent mobile monitor, I would consider it exclusively for a travel rig.
I've got an external monitor and my full keyboard and mouse with a dock and my steam deck. I can set up anywhere with a desk and game, program, whatever. I've found very little that it can't handle.
I'm using this portable monitor but it is kind of finicky, and I worry it'll break easily. It's the biggest one I could find at a reasonable price, and happens to fit my backpack.
I've used it with my 3440x1440 freesync monitor at home, and it works as well, but like anything, whether you can game at that resolution is very dependant on the specific game and settings you use.
I'll add, because the deck doesn't have thunderbolt, plan on using HDMI instead of type c.
It's possible you can find a type c (non-thunderbolt) dock that supports powering and driving a monitor over type c along with the deck, but I wasn't able to find one.
Outside of the better gpu, the one advantage the other devices have is emulation. Steamdeck sits on the edge of performance for some of the harder to emulate devices heavy titles (PS3, Switch). The ones using Ryzen 3/4 would trivially handle emulation better than the Steamdecks CPU, which uses Ryzen 1+ (part of the reason why its low cost)
I am looking specifically for a single device for travelling with. But the built in controllers of the Steamdeck are just a little too goofy for me to give it much serious consideration.