My guess as to why there isn't a user-accessible Snow Mode setting with a 50-50 split is that it is too power hungry (especially in cold weather). They don't want people to turn it on, drain the battery, and then get stranded or pissed because of the range hit. So they added
that trigger (whether it's slip combined with temp/camera imagery or just slip with temp) to split power until the wheels seem to be getting enough traction, then slowly transferring power to the rear motor entirely.
Crazy speculation time: I believe they'd love to go 50:50 split more quickly and/or more often (or offer a Snow Mode toggle), but need the efficiencies of the 4680 batteries to make it feasible. Like I said, bananas crazy speculation.