Heavily depends on the suppliers and contracts Nintendo has with them. For all we know, there might have been a higher grade version but it ran into development issues and was delayed/scrapped for all we know (or they settled on the version we have now)... or the higher probability reason is that the demand for a higher spec version just isn't there to justify the cost of development at this time. I mean it's not a big secret that the consoles themselves tend to not be the big money makers compared to the game sales themselves, and a chip shortage would likely make that gap grow.
Realistically, when it comes to the Steam Deck, I don't think it'll be as big of a hit compared to the Switch. As someone already stated, getting your Steam library playable on your TV is already super easy, and getting it to your mobile device is possible. However, the big issue comes down to how games are designed. When considering the Switch, the games available to this platform are specifically designed (or optimized) to run on the Switch hardware, so anything available to you on the Switch has a very high chance to run well and be playable. However, the Steam Deck does not enjoy this functionality, as you're accessing games designed and optimized to run on a PC... and in some cases, the games are designed/optimized to run on high level hardware. This isn't even going into whether a game is designed to run with a controller or a keyboard/mouse setup.
The end result is that you may be extremely limited on what you can feasibly run on the Steam Deck from your Steam library. While the specs of the machine seem decent, it's not really telling us the whole story or indicating how well it will actually perform. The major hurdle that they will likely run into is overheating and throttling, which we have no information on (and likely won't until it releases and people actually test it). Furthermore, the game titles themselves will likely have huge variance in how fast they eat the battery charge, how hot the hardware will get (or how fast it reaches thermal throttle thresholds), and their resulting performance. Heck, there's a ton of games in my Steam library that are poorly optimized and eat tons of resources on my high-end PC... there's no way it'll run well on a hand-held device.
If I had to hazard a guess, I would estimate that the games will run at 40-50% of what they would on an actual PC (similar to the performance difference between laptops and PCs using the same generation chipsets). For some games, that won't matter at all. For some games, that's just unplayable. The resulting market will be for light games or older titles, and at that point I don't think there will be that big of a market for such a device... unless we need another platform to play Skyrim!