Why are the stances so low?

That's definitely part of it. The relatively constant reinforcement to "get down" or lower yourself is important - it sinks weight and facilitates the release of tension.

 

People tend to use a higher stance under pressure. I get my students to train lower stances so that under stress such as a tournament, they posture up to something moderate.

 

In wing chun, we usually try to train how we fight. I personally don't like to be too low because of mobility issues even though it might be more rooted and balanced --- especially since wing chun relies a lot on moving into the opponent.

But yeah, in other arts, it seems like they use it for strength training.

 

One time I sparred with my Karate opponent, I used a very low stance with strong defense. I moved in inch by inch. Since my opponent could not find any opening on me, he moved back. Soon he lose desire to spar with me. He was good in high kick but apparently he was not used to low kicks.

All wrestlers use low stance.

- Less attacking target.
- You hands are close to your opponent's leg.
- Your center is low with good balance.
- ...

When you use your right foot to hook behind your opponent's ankle, at the same time you try to use your left hand to pull his ankle, you have no choice but to stay low.

 

Pigeon toed?

 

Something I was told back in the days of DSD Kung Fu, and something that makes sense to me, a lot of low stances are generally incredibly unfamiliar and awkward positions for one to go into that allows your body get more familiar in uncomfortable or unatural stances/positions. We would often do things like stay in low stances rotating around and attacking/defending from that position. It would get easier, it would get faster. Later we would adapt this in boxing/kickboxing style sparring situations. Things like pivoting, footwork just seemed more fluid from a "normal" fighting stance or situation where we were taught to "adapt". The mentality was if you could fight/spar in those positions where you're uncomfortable and it's difficult in a normal situation you're more prepared and it's easier.

 

Our beginning exercises is simply stepping forward, side and back (plus diagonally) while aligning the body how it is correct for our system. Here we are told to drop low, for the same reasons others have already expressed: strengthening the legs, preparing yourself for advancing low for throws, being low now so when in a stressed situation you moderate and are not in too high a stance.

I like working this way but it does bring about a good burn in the legs.