The difference is that Steam Deck is actually cheap compared to what the competition does. It's also the first generation of Steam Deck and the upgrade with an OLED (and lot of other stuff too) is actually substantial. And there are multiple versions of the Deck available to choose less drive space. Imagine this was an option on PS5 Professional too. Contrary, the PS5 Professional is the most expensive console compared to its competition. It's so expensive, that it set a new bar.
That's the opposite of what Steam Deck does. Steam Deck is the only current generation game console that gets cheaper over time. Also one is a handheld format, which is hard to make cheap, especially because its compatible to PC hardware (and software).