Are you drilling 1/8" holes or driving in small screws a couple of times a year? The batteries that came with your tool should last many years.
Every type of user wants less expensive batteries and more opportunities to save money.
What about consumers who use cordless blowers? An increasing number of towns and cities have ordinances banning gas engine blowers.
Cordless chainsaws are increasingly popular with homeowners, and their power requirements can be very different from those of a cordless drill.
Every brand now has cordless trimmers, mowers, and more.
A universal interface would all but require most users to start having to geek out over battery selection unless they stuck with their tool brand's options.
Even if all the technical hurdles are knocked out of the way, what's to stop brands from designing their tools to only deliver their full potential when powered by their own batteries? At least one brand already does this.
So you might not care about geeking out, but what happens when you buy the wrong battery and your cordless mower can't cut tall grass when it can with the batteries it was sold with?