are you saying why don't they only make 2 ohm speakers??
What if you have 2 2ohm speakers, well now you have 1 ohm... are all amps stable at 1 ohm, no??
What if you have a 2 channel amp and want to use all of its power bridged?? Well then you need a 4 ohm woofer because most 2 channels can't bridge at 2 ohms.