Because every server an email passes through will at least add its own Received header, and every antispam, DLP and antivirus crap appliance in the chain adds another one or more.

Usually, you have 1-3 Received header from the sending email organization (record I've seen is five - three ordinary mailservers and two for AV and DLP crap), then you got Proofpoint adding 2-3 layers, and then MS Exchange or whatever adding 4-6...

That's why the sender only signs those headers he himself is aware of and sent out.

Oh, and then you got the even worse cases, email servers actually modifying the body to add "Inspected by <Bullshit Snakeoil>" footers.