But in real life there aren't as many ruins in one location as in Cyrodil. I think the presence of all these forts show how the importance the Imperials gave to military might.
They're a reflection of a bygone era where Cyrodill/the Empire was much smaller and besieged on all sides by enemies, even within its own borders.
By the time of the events of Oblivion, the Empire is well established and most of the remaining operational forts are within established cities or in provinces like Morrowind or Valenwood.
A perfect modern day example would be the old abandoned castles and forts that litter Europe. It's something you don't see as much in other places in the world - North America for instance. That's because of the unique history of Europe. Those forts were built during times of conflict and uncertainty, but in today's relatively stable climate they're simply relics of the past.
The abandoned forts in Cyrodill are the same way. They used to be fully garrisoned at different periods in time, but up until the Oblivion invasion, Cyrodill was peaceful and stable and had no need for manned forts. Most of the soldiers stay in the cities, etc.
Forts...well the Oblivion crisis would obviously make the Imperial Legion abandon the forts and instead garrison themselves inside the city or the minuscule forces, obviously from reassignment, would be overtaken by the huge bandit armies. One exception is Fort Sutch, which seems to be one of the more important forts as the Imperial Legion tries to close the gate. Failing, but still trying nonetheless. Another theory is that the forts were simply useless as Uriel's reign was a peacetime.
There's two theories about Sutch.
One is that it was originally meant to be a city or settlement in Oblivion but was left our and turned into an abandoned fort at the last moment. So the hidden quest to defend it is a leftover.
But I've thought up my own semi plausible explanation.
I'm assuming they're protecting that location not because of the fort but because it's a critical pathway for scouts to carry news between Chorrol and Anvil. For the Daedra to overrun and occupy that strategic location would cripple the ability for information and goods to travel quickly through an off road path. In times of war, protecting such routes is an important strategic element. So whether or not it's stated out loud, that seems logical to me and that's what I choose to believe.