Why do we need to install a power transformer for each high building?
Well, if you didn't have a transformer how would you convert the relatively very, very lethal 11 kV into the definitely less lethal (but still rather lethal) 380/220V?

If, on the other hand, you meant that somewhere down-town you converted 11 kV to the relatively-safer lower voltage and fed this to all the high rise buildings then, the cable losses would be massive because the currents would be 50 times higher.

Or maybe you're thinking that consumers convert all their appliances to 11 kV - that just aint gonna happen for reasons of safety (irrespective of the massive global cost).

Another half-reason is that the 11 kV network is transmitted as 3 phase balanced i.e. it doesn't have a neutral connection: -



It's transmitted as 3 phase due to economies of making 3 phase generators in the hundreds of mega watt range. But, households in the UK require a single phase and neutral and this is from a historical safety standpoint (in the UK and Europe at least). To create a neutral wire requires a transformer (delta to star configuration) so there is also the cost of extending the neutral wire if only one transformer fed several high-rise buildings.

Also, one fault in one building might trip the whole building out but it wouldn't trip a whole precinct of buildings out. Same is true from a maintenance standpoint - it's less disuptive to isolate a single building than a whole precinct. 

I would also imagine, with high rise buildings (subject to lightning strikes) that having a local transformer for each building will be less disruptive to other buildings not hit by a strike.