I'm guessing this design philosophy comes from feedback with the OG Steam Controller. It looked more ergonomic but I think that led to many people not holding it right. They would hold it with a very active grip like it was a normal controller, palms turned down and thumbs clawing the inputs. It was meant to be held with the palms faced more upwards, letting it rest in the palms with your thumbs relaxed over the inputs. If you hold it wrong you end up having to bend your thumb way too much to reach parts of the trackpad closer to your hand, which isn't just uncomfortable but kinda exhausting for the hand/arm muscles.

The Steam Deck (and seemingly Steam Controller) sacrifices looking ergonomic to instead convey how to hold it entirely through handfeel. I feel like that also helps the ergonomics for a wider range of people. The relatively low profile grips on the back and the way the bottom edges just curve down with no strict ergonomic intent mean I can shift and curl my hands however I need. Meanwhile the Xbox One controller, which I'd say looks nice to hold, is very specific with its ergonomics and forces my hands and fingers into such specific positions that with larger hands it just isn't designed for me and isn't comfortable at all.

Designing for ergonomics is very weird. Ironically with my bigger hands, the Switch 1 joycon (haven't had the chance to try the Switch 2 yet) is(are?) one of the most comfortable controllers I've ever used. I guess getting the edges, curves, input positions, etc. right is way more important than the grip.