Why don't more fighters fight on their toes anymore?

[pacfan]

... A serious power punch comes from a flatter foot stance.

Right. Power punches must be firmly anchored and that anchor comes from feet firmly planted on the ground. You can still throw power punches even while moving if you can sort of lean on those punches when they land on their targets....
...but...
...you risk losing your balance on the way, especially if you miss...