Because indie games are popular. Old games are still games. Steam deck has a huge library of games that can run. They arent trying for that flashy AAA market.
Look at Nintendo, switch is old and gets lots of complaints about it being low end hardware. Yet, people still make great games on it and their is tons of support. If anything, Valve is emulating that.
Also, I case you didnt know this, supporting many different machines with drivers and configuration types draws a lot of resources. My guess is Valve wants to work on the compatibility later before even opening SteamOS to a wider audience. Supporting one machine allows them to give great support.
Cheaper machines help keep churn and maintenance down. High-end cost more to fix/debug/maintain. Lower profit margin means even less support, like Asus (shots fired)
But also, Valve wanted to keep it as simple as a Nintendo switch. The more they can convince people to use Linux, the more security they can buy considering Microsoft is changing windows. Some of those changes could directly affect their bottom line, like closing off their OS to anything but the Microsoft store. Microsoft would love to get the revenue that iOS or Android app stores get.
Anyways, those are a few reasons Valve keeps things simple and straightforward.
Since the colors have picked up quite a bit, I can chill on the OLED. But the wifi would be nice for remote play.