Because of the overheads involved, that's why not. You can get the same office space much more cheaply elsewhere. Heck, if you're worried about people not wishing to commute for 100s of miles just go to Stockley Park or somewhere sarf of the river, or Ilford. Anywhere is cheaper than the City. Since the cost of the space, like all other overheads, will be shared among the participants it makes sense to go for something cheaper.
Making a sunk cost argument to hold on to expensive space under lease is a fallacy anyway. Sublet it to someone else, then take cheaper premises. Easy.
So, to sum up:
A clearer finds itself with too much office space. Rents it out to captive arcade at high rates, thereby ensuring that that non-profit company doesn't make a profit, but the clearer does. Strikes exclusive flat-rate deal with said arcade to do its clearing (i.e. not passing on exchange discounts and incentives), thereby ensuring that the clearer makes a nice profit again.
Nothing wrong with making a profit, but this won't wash. If your major selling point is low cost and overheads you don't start by incurring high overheads and you don't forgo discounts and incentives.
would be more like it.