Why did Goku stay on the mountain until Bulma came?

[Kamiclouds]

So until Bulma came he’d never left the mountain that Gohan raised him on. He seems like a fairly adventurous person, and he knows that there’s a world out there but he tried to leave. Goku has never been one to be afraid of change, so it was always kind of weird he never wanted to leave.

[Koitsukai]

He was 12, and had all of his needs covered, so he had no need to migrate somewhere else. He had no idea about the real world until Bulma came along and told him about the DBs and the adventure she was on.

He even thought he was still living with his grandfather who had magically turned into an orange sphere.

[Saiya6Cit]

Somehow I think Granpa Gohan had something to do. He knew goku was a strange kid and unlike the parents in smallville who sent Clark to school, gohan knew he had to keep Goku isolated, this is he never even took him to a festival in a near town or to even just see other people. Granpa Gohan thought he was a freak.

Also as koitsukai mentioned, he had all of his needs covered, he had shelter, food and Gohan taught him to dress himself so probably also to make clothes or they used to belong to Gohan and left them for him (?) so Goku was not entirely a monkey boy (despite of the tail) because he lived in a house unlike Tarzan, but he is still close to jungle kid.

[Demon Prince Piccolo]

Believe it or not, Goku was actually pretty sheltered in a sense. Well, "sheltered" maybe isn't the right word (that's more accurate to his son), but he was living by the skills Grandpa Gohan taught him in an extremely rural area. When Bulma shows up and uses the hoipoi capsule, he immediately assumes she's a witch and seems afraid. So it's clear he isn't used to anything outside of his woods. He's not "afraid," but the familiar is safe to him, and everything else is an unknown and thus unsafe until he goes exploring with Bulma.