As far as I see it, they have to spend shitloads of money sometimes, but there is no incentive to spend money on improving and adding features to the core systems such as the ledgers, as it is very hard to get a return on the money that way, partially because it will not really improve their market share.

The money is spent on systems on top of the core systems (web etc) basically trying to stay on par with the competition. If they spend on the core systems, it's when they try to consolidate and save costs.