Because of the inherent mechanics of killing in the game, obviously. If no one gave any hints, mafia would default win. We aren't giving hints because it's fun and customary, but because a hint is as much as we can give under usual circumstances without receiving mafia death penalty. Those who don't participate or are suspect don't give out their roles out just like that either, because a hint is maybe enough to both prevent being killed and lynch.
Impressive math skills, but you're being contrarian for the sake of being contrarian (which makes me a bit suspicious). On a day with zero information provided, having a 30% chance to hit mafia members is actually not too bad. It becomes worse as the game goes on, that's so, but I think if we go with your logic, it would make most sense for town to completely stall the vote, try to not lynch anyone at all till we have a 100% chance of hitting mafia... which will never happen (because we will never be ahead in info, the game is designed that way). If you can see the merit of that reasoning, you can also agree that this power transfer thing is basically the same as democratically deciding we're okay with another day time lynch, which is what we should be happy with since it provides us with more agency than usual.