I imagine their friend conversion rates are as low as mine if not lower, but they are approaching about twenty times as many people as I do.
Absolutely.
There are regular threads about friendships on MN, and it often comes up that some people just get up, go to work, come home, and stay home - which is fine if that is what you want to do, but if you want to increase the number of friends you have, you have to meet lots of people.
but I found these friendships seemed to have a shelf life
I think many friendships do. They are "of a circumstance" - school friends, university friends, a friend you commute with, a friend you work closely with, a friend you sit next to in a baby group or a friend you walk up to school with as a parent, etc. Only a small % of those will develop into friendships that are longer than the 'circumstance'. As you get older, and look round at who you are still friends with, after the circumstance has gone, those become your lifelong friends - and sometimes remain friends when you don't see them for several years.
Friendships where you see people every day or every week are, of course easier than friendships where one of you moves away, or when one of your circumstances change (eg one has a child when other friend is young free and single).