Because of the convenience, and because Apple and Google have managed to convince users that it is safe enough for them.
Most people only encounter biometrics on their smartphone, where it prevents unwanted access by others, and acts as a master-password. Most people feel that they are never going to be the target of anyone capable of bypassing the biometric security of their device, and are more then happy to make their fingerprint or face the key to their castle.
They also don't really have any alternative. Entering passwords is a chore on those devices, and the PIN or swipe pattern is way too easy to observe by a would-be adversary (which often includes family members or others near to the owner who want to explore (saucy) photos taken and texts sent).