That was my impression as well but now I’m puzzled about why. Since most smartphones have GPS builtin why not get time from GPS?
Mobile basebands already get the local time and UTC offset when connecting to a tower, there's no need to power up the GPS antenna and receiver separately.
Mostly because 1) your phone is getting time from the tower anyways (they have to be closely sync'd) and 2) GPS doesn't always work very well -- especially indoors.