Switch poles from inverted power to make negative positive?
That is fine so long as the signal/power is isolated from the common reference/ground. In fact, using an isolated DC-DC regulator, it is common to swap the output leads to get negative voltage.

The problem is that generally, one side of everything is connected to a common reference/ground. For example, on an ATX power supply, the negative side of +12V, +5V, +3.3V are all connected together on the black wires. If you try to reverse the +5V rail, for example, you will end up shorting out that rail.

If you have a signal or power source that does not reference the common ground, you can easily invert it as you suggest. The problem is that it is often harder or more expansive to have these isolated.