Humans are social animals and friends are needed for a healthy psychological balance. Is it a trust issue?
Probably not a trust issue -- a comfort zone issue + how their life rolls. Many people when older don't because they are content (for better or worse) doing their job/work + dealing with what they do at home, and reference to family stuff. It's not unusual. When one ends up getting free time after periods of time in their life where they didn't have much Time for friends let alone make any new ones -- they end up instead just have time for the open dating scene if single.
Would you date a man who chooses to have no friends?
Dating men is not by bag, baby. Would I date a gal who has virtually no friends? Sure. Although the optimal happy medium is that she does have some friends (not always lonely), but not part of a gal-pal group that ends up affecting each others' relationships with dudes.