Because while a self-inflicted Total Party Kill has its charms, it’s very wasteful to let it happen in the first couple of game sessions. You need to let them build up to it on their own. So that it’s personal. Cathartic, really.
Annnnnnd that’s why GMs still do the ‘roll to see if you see the panther in the tree‘ line. Or, rather, sometimes why they do it. It’s a handy hint to get people’s heads back on a swivel.