Why does NAND erase only at block-level and not page level?

If you don't wipe them all at the same time, you'll need a much higher voltage because you're trying to raise the floating gate voltage a certain voltage above the source voltage. If the source isn't tied to ground through the other transistors, many of the source voltages will already be at some level higher than ground. Furthermore, if you tried to use a higher voltage, some of that voltage would likely end up on some transistors with their sources tied to ground which may be enough to damage the transistor.