Author and researcher Arthur Brooks had a fascinating take in a conversation with Dan Pink (~3 min video). Below is my paraphrased version.
The reason is not the number of friends. It is the type of friends. Specifically, do you have real friends or deal friends?
Deal friends are the people who can help you in a transactional way and they probably do care about you, and you probably do like them.
Real friends are the people that might call you at 2 o clock in the morning in a crisis. This is somebody that you might share the intimate details of your romantic life with without feeling weird about it or without feeling it might be divulged to a whole bunch of people.
Think of the 5 people you are closest to. Now ask yourself – “How many of these are deal friends vs. real friends?“
A simple way to find out – deal friends are useful to you to further your career. Real friends are “useless.”
So, if all your top 5 friends are deal friends, good for your career. But, it also means you need to start doing the work to find real friends.