In IP header we have header checksum field which is calculated at every hop because some of the fields in IP header like TTL changes in every hop.
In TCP we calculate checksum on pseudo IP header + TCP header + TCP data . Here pseudo IP header doesn't contain the IP header fields that change frequently.
So in IP header checksum why don't we compute checksum on pseudo header or a header excluding fields which change frequently like TTL ? It will also help to reduce the computational overhead .