probably to save money. that sucks, because even if you swapped it with another defective one, it would be worth it if you just had to replace the CPU every 18 months.
Learned in another area of this post that yes, they are soldered on, but that's Intel's choice. They only offer the CPU in a BGA (ball grid array) form factor. There's no such thing as a BGA socket, other than some specialty test unit things that aren't suitable for real world use.