I think the real weirdness is that since everyone hits level 9 pretty much at the same time, the DM has to pull five excuses for everyone acquiring real estate out of his ass at the exact same time.
"Yes, everyone's rich old aunts all died at the same time! Now can we move on?"
First off, in AD&D, characters did not hit name level at the same time - AD&D had different XP advancement for each class.
Also, it wasn't up to the GM to pull something out of his a**, it was up to the players to decide how their characters developed these strongholds. My own experience:
One fighter hired a ton of dwarves to build his keep. The main retainer was the castellan, and the others were primarily guards and a rapid reaction force.
Another fighter chose to forgo a keep, instead quartering in the other fighter's keep. This second fighter was also the main general of their combined forces.
Two clerics each established temples in different areas, each with guards, novitiates, and lay brothers.
A wizard established a wizard's school in a town near the fighter's keep. He started acquiring his library several levels before name level.
The thief took over the thieves guild in the same town, cleaned out the rougher elements, brought in new recruits, and established a continent-wide spy network.
None of it was given to the characters by the GM - each player/character made choices, and the GM determined the consequences of those choices. (For example, during a war, the fighter's keep was burned to the ground by a lich, who was later killed by a wizard PC.)
You get out of the rules what you put into them. We got a lot of good gaming out of the keep rules.