Because it is a mobile console, therefore battery life is a limitation and adding an extra layer of indirection (and therefore, work) will drain that battery faster.

I hate to break it to you, but battery life will be at the bottom of the list of Nintendo's concerns when giving you backwards compatibility. If Yuzu was able to get 2.5-3 hours of battery life on the Steam Deck (which isn't that far off from what it gets playing a lot of "native" games) essentially flying blind, Nintendo should be able to do at least that.

4.5 to 9 on the actual Switch with a 4310mAh vs 2 to 3 on the Steam Deck with 5100mAh doesn't seem to prove your point.

The Steam Deck also has a significantly larger battery.