Even if they do standardize on vendors, they're then also dependent on, say, Oracle Retail supporting a given platform.
If you want your POS to talk to your marketing automation system, that's another integration and maybe another vendor who Oracle may or may not wish to support etc.
There's no way for a bank to standardize on vendors as it's back office systems might have been designed in the 1980s. If it wants to add an iPhone app or mobile payments, it has to rely on another vendor almost automatically.
Making these stacks work generates huge revenues for people like CA, Automic, IBM. Process automation is big business. Big meaty huge Fortune 500 companies held together with the software equivalent of sticky tape.
Not to be crude but what you call "standardizing on vendors" I call a great way to spend your days getting screwed by said vendor(s).
Also, they should pay the vendors contracts for maintenance instead of trying to do the upgrades themselves. The vendors are generally more likely to do the testing necessary and have the skills for upgrading systems across from various customers.
At any rate, as you put it, eventually they still should do the upgrade.