Because it adds to the software complexity. several boolean toggles for deciding what to incorporate into the localization algorithm: one toggle for GPS, another for wifi, Bluetooth, cell tower, barometer, etc. and fusing these in all the various combination, testing them, etc.
and after doing all that users will mess up their config and then complain that this app sucks not knowing someone on HN had this feature request for "privacy" reasons even though it's pretty unclear to me what privacy is being gained here.
Apple products generally don't give the user much choice and decide what's right for the UX and it's served them well