Modern TCP/IP stacks have a lot of extra code, including for anti-spoofing, performance enhancements (eg zero-copy integration with hardware network cards), various attack prevention measures (SYN floods, randomization of sequence numbers, etc) support for various hardware offloading (including many network cards that will do checksum offloading, etc), IPv6 (that also originally mandated IPSec integration), support for lower layer 2 protocols (mostly just ARP for Ethernet, but there are still others around).
