However, I've learned that although sending Bitcoin from a legacy address to a SegWit address isn't possible but it is possible to send Bitcoin from a SegWit address to a legacy address, and I haven't figured out why.
You are wrong.
You can send bitcoin from any type of address to any type of address you want. This means that you can send bitcoin from segwit address to legacy address and vice versa.
Well that's Strange, cos I know I tried it sending Bitcoin from a Legacy address to a SegWit address, and I'm pretty sure it didn't go through.