It’s not, it’s just that forests are really bad for phone signals. Trees disrupting the signal, valleys and sides of mountains with no direct view of antennas and just remoteness in general. I’ve been in a lot of forests with no cell reception, that’s why you always go with an offline map and tell someone where you are going.
Yes but still I’ve had very good results even on 4G in large forest patches while hiking and whatnot.