I've also thought this....but not exclusively because of no available land. Because I don't think they can handle 40k in one server with good performance, yes it has gotten better but they've never cracked this issue on Ark and it's been out years.
They calculated 40k from assuming there is an equal distribution of players in each grid...that will never happen.
There is also no enough landmass in the game, a small group can take over a grid easily (if is was available)