You have to have the transformer from 11 kV to 380V somewhere. If it's far away from your buildings, you have substantially increased transmission losses. If you had one big transformer close by all the buildings, those losses would be substantially mitigated. But there are still a few disadvantages.
One, transformer secondaries are often neutral-grounded to building steel, and each building can have a slightly different ground potential. With one big transformer, you'd have one ground, which would be different from the steel in each building. This could cause grounding issues.
Another (just to make up some numbers), moving ten 1 MVA transformers around is probably easier than moving one 10 MVA transformer, and the transformers themselves may be cheaper as well. Economies of scale and all that.
Third, the single huge transformer can't just sit outside. It has to be enclosed in something. Why build a purpose-built structure or expensive outdoor-rated enclosure, when you have perfectly good buildings available?
Fourth, maintenance becomes problematic. If your transformer ever has to be replaced for any reason (admittedly a rare occurrence), now you have to shut down all ten buildings instead of just one. And since each building still needs its own disconnects from the grid, you still need a lot of room in the building for breakers and switchgear.
Fifth, having separate transformers helps isolate each building from line disturbances in all the other buildings. If someone sticks a fork in the light socket in building A, or everyone in building B turns on their vacuum cleaners simultaneously, building C will see all that line noise if they're on one big transformer. Separate transformers help filter a lot of high frequency noise from passing between buildings.