It's hard to beat the convenience of continuing to hear your music seamlessly from home to car, or the car knowing where your next calendar appointment is and having the map ready to navigate you there. That, and the fact you upgrade your phone a couple times faster than you change cars, and on every phone iteration, the in-car systems will appear increasingly sluggish in comparison.

This is also why modular radios/8-track/cassette/CD players were popular - you could upgrade your sound system without upgrading the car.