Well, it makes the hardware more complicated to manufacture, more error prone, more bigger, more costly. The software becomes also a little bit more complicated. Maybe they are afraid that trackpads wouldn’t appeal to normies, because it looks weird first and most “console” gamers might want to use a “normal” gamepad like handheld, like any other handheld. Trackpads look so different and weird compared to the Switch in example. Being similar to the Switch in the look might be a reason for them to exclude it. But most importantly, it makes it more costly. So, I can see reasons for this.

Having said this, I would never buy a PC handheld without a single trackpad again. I love them (not in the literal sense BTW :D). Compared with good software for input, like Steam Input, they become invaluable.