The fact is: fleets take up database space. The purpose of a fleet is for a group of players to form a sub community. If every player had his or her own fleet, it would increase database overhead per character exponentially. It's also the reason why a player has only a handful of character slots, given everything that is stored for single character (ships, BOffs, DOffs, inventory, bank, mail, mission logs, etc.).
If you are going to try to advance a starbase with one person, it would be so prodigiously slow as to be ineffectual. You'd be lucky to make SB Level 2 in a year solo, never mind trying to get a Tier 4 Shipyard to unlock certain ships, or ranking other tiers to open up starbase contacts & vendors.
And I've seen plenty of MMOs "hide content behind a guild-only door". DAoCamelot, EQ, EQ2, WoW, CoH, CO...
There are still plenty of fleets that will let you join them, asking little in return, and granting you access to all a starbase has to offer...
I guess you have not heard about solo fleet "Starbase 001" - the first solo fleet to tier 3
Currently tier 3 Sci upgrade is done - in 2 days the Shipyard will come off CD and be finished and then the Tier 3 Eng Fab will be launched - so in 10 days the final Tier 3 upgrade itself will be launhed
Within 17 days from today Solo fleet "Starbase 001" will have completely produced a tier 3 Starbase