Because we don’t put in enough effort to maintain those friendships.

The society is also selling us constantly the idea that you can be self sufficient, that you don’t have to depend on anything or anyone even friends. 

Aside from that a lot of the people we meet are not our friends, they are just people we called friends because we had common interests and circumstances, so when those interested fade away, or those circumstances change, those friendships die slowly. 

(Sorry for the long answer)