I can kind of see your point, but take a look at Tower 42 and see what kind of companies are in there. NatWest have gone and now you have different small companies on each floor - so it's not like it has one company with all of it's employees in one building. Apart from financial institutions no one seems to be interested in that here.
Most multi national blue chip companies like Microsoft, HP, Oracle, Vodafone, Three, O2, Orange etc all seem to prefer having their UK (and in many cases European) headquarters outside of the city but within commutable distance. Why they are all based in the M4 corridor when they could be in the city? I guess one of the reasons is that it's far easier to get to Slough or Reading from Heathrow than it is to get to central London and having a nice 'campus' is a lot preferable to most employees than being stuck in a high rise block.