Why in the world would the tab on a TO-220 package have a connection to anything (but GND, if applicable)?
Why not just leave it disconnected?   


Because of the way the IC is made. It starts off as a (relatively) large piece of doped silicon, and the features are deposited / etched / implanted into one side. That leaves the back side of the die electrically connected to one node of the circuit. Which node it is depends on what circuit is being made. Generally for things like ICs, it's the most negative terminal. It is whatever makes the circuit the most straightforward to design. 

For a power device, the designer also wants the lowest thermal resistance from the active region to the case. The most critical part of this path is the narrowest directly under the die, which is why a direct connection is used. 

It's far better if an insulating layer is needed to put it between tab and heatsink, where the area available for heat conduction is orders of magnitude larger than the area under the die. And remember not all applications will need this insulator, some can use an isolated heatsink.

The bottom line is cost/performance ratio. An insulator in the package would increase cost and decrease thermal performance. There are niche applications that would pay that premium, and there are some isolated tab power electronics out there to prove that if you look for them. But most equipment manufacturers will do the extra bit of mechanical design to get the best overall cost and performance.