My experience has been that most 18Over/23U teams don't form until mid May once their collegiate season is over. While a lot of players today may say that they will play next year, that often changes after the player has gone through a long spring season. They simply want a break. When they do play, they don't want to spend every weekend playing and teams often don't get enough players to commit to any specific weekends. Even after commitments have been made, players will drop out, often leaving their teammates struggling to find a replacement.
When they do have enough for a team, the 18U/23U tournaments are great--the parents are great, the players don't get upset over losing or making physical errors, they realize the umpires are out there trying to do their best but that do make occasional mistakes (when this happens they just give the ump a look of disbelief--this is more effective than arguing).