I could list dozens of reasons but at the end of the day it comes down to having the option and making the choice, instead of having it made by some company. And as someone who repairs their own phones, 3.5mm jacks really don’t take a lot of space. It’s definitely not enough to make a substantial improvement to battery size. You question the point of having them, I question the point of removing them when they were (and still are) a universal standard with not many downsides.