Why do they design the seat belts that way?
They fit adults better - stay low across the hips, less likely to ride up on the abdomen.
There's an interesting dilemma that vehicle manufacturers are going to have to address in the very near future... they're continuing to make vehicles safer for passengers, but they're not considering how increasing the safety to adult passengers interferes with installation of child safety seats. Many of the things that improve safety to an adult passenger, cause problems with car seats & booster seats, so this is really quite a big issue... The back seat is designed for 110lb adult, and it's great to find way to keep them safer in the vehicle... but most people buy larger vehicles because they have families and they will be using carseats and/or booster seats for many many years.
And this is why, when parents go car shopping, they should take their carseats with them and test them in the vehicle before buying - unless you're prepared to possibly replace all your seats or lose seating locations in the vehicle that you thought you had. (And read the vehicle manual before test fitting the seats, sometimes important info affects compatibility of a car seat & it's always good to know that beforehand instead of just thinking the car seat isn't going to work...)