X.509 CSR: Why does CA remove signature? I just read this article on Wikipedia: Certificate Signing Request

I'm not a PKI or Crypto expert. As I understand, a CSR (certification request) is always signed by the PKCS#10-Request creator. 

What I don't understand is why the CA, which receives and processes the CSR, then removes this signature (I mean, without this signature a CA could possibly alter attributes of the CSR). 

Why does a CA not just add the necessary attributes to the CSR and sign it with its private key?
If the CA issued something with a CSR as the dominant part of the To-Be-Signed field, it wouldn't be a X.509 certificate and hardly any existing software would know what to do with it. I guess the original CSR could be added as an extension, though.

Therefore, I suppose you are really asking why the X.509 certificate format wasn't originally specified to include the signatures of both the subject and issuer. I guess the reason was that if a PKI application trusts a specific CA, then that application trusts that the CA adds correct information (pairing a subject with a public key) in all of the certificate fields before issuing it. Adding the signature of the subject wouldn't add anything security-wise.