I think there was a choice by the writers to not give Monk more friends. There were episodes where someone was nice to him and they were never seen again. For example Monk and the Employee of the Month when he he helped his former partner and the college reunion episode when he reunited with Trudy's roommate. Trudy's parents seemed fond of him yet they were only in one episode. It's no surprise he was so dependent on the friends he was allowed to have.