Why Does BHVR Keep Doing This?
I think having cosmetics for buying a Chapter directly is neat, most are just slight reskins of default cosmetics so people might not care as much about them, and they can be obtained at any point and are not limited-time, my issue with them just we never really see cosmetics that killers can use. And for someone who plays both sides, it's a bit of a bummer.
Outside of charms, the only killer cosmetics you get for buying a Chapter (as a far as Im aware) is Ghostface's white knife and shroud, but even then, it's a Chapter that only has a killer to begin with. I kind of wish things were a bit more even for both sides. It doesnt even have to be much, a reskinned weapon will do, just something at least.

Comments
Killers already have something unique to sell them (their power), while survivors are just cosmetics.
Also, I think it has to do with being able to actually see the cosmetics in game. As survivor you get to see your cosmetics almost 100% of the time. With few exceptions, as killer you only see your cosmetics right at the beginning of the match and then in first person you only get to see hands/weapon.