Bachelor,
That's because the subwoofers you've been looking at are designed for stereo music systems, not home theater systems. You don't mention manufacturer or model numbers, so they might be optimized for high end audio systems or they might just be older designs.
I suspect they also have L&R outputs. Models like that are intended to go between L&R preamp outputs and L&R amplifier inputs. Some include a built-in crossover network, others need external crossovers.
Anyhow, they should work fine when connected to the subwoofer/LFE output of a home theater system, either with a Y cable or with just one side driven.