Outsourcing to somebody that can drive higher volumes for components can be cheaper, as they can amortize the manufacturing equipment over more widgets. If a car seat is just a car seat, then outsourcing makes sense. However, if you need to customize it or add some other value (heated seats, electronic movement, etc) then unless everybody is doing the exact same thing, you need to bring at least some of the components in house and/or pay for it.

The same is true in many industries. TSMC has dominated fabing chips because they scale wafers over many more customers, whereas intel mostly only focused on its own chips. The results was more expensive all around. (Intel also had to deal with bad western accounting attitudes to CAPEX spending, which IMHO is a huge reason so much manufacturing left North America, though lower costs also played a role).