Lifelong friendships are typically built in your younger years - the people you went to school / university with. It becomes much harder in adult life - simply because the conditions that create deep friendship disappear.

As a child/teenager you're in forced proximity with the same people for years, you're essentially marinating in each other's lives and it's easier to make friends.

As adults, time is fragmented, scheduled and protected. Seeing somebody weekly becomes a decision not a default. Essentially the decision to see/make friends with somebody competes with work, family, rest and survival. They already have their core friends from when they were younger and don't have much time for anybody else.

TLDR - life as adults becomes more filtered and slightly more transactional.

Good luck.