Right. My sister is always having a hard time with math and it seems like the teachers are always focusing on solving equations rather than explaining the reasoning behind it.

It's pretty simple when you realize it's (mostly) all about lines.

This is the biggest contributor. Teachers focus on making sure they follow precise steps whenever trying to work through a problem. While that's helpful for certain things, if the student can explain why they are doing what they are doing then they won't be able to apply what they've learned to solving new problems.

It's definitely not an easy problem to solve and I certainly don't envy elementary/middle school teachers.