The consent and the data collected via the consent need to be linkable. That's why it makes sense to store consent records for identified users on the server-side, because you "know" the user in that case.

For pseudonymous users, e.g. those you track via a Google Analytics cookie you don't know who the user is and you (hopefully) can't reidentify them without the Google Analytics cookie. Since the cookie is stored in the users' browser it makes sense to also store the consent record there. If you would store that consent record on the server-side you'd still need a cookie in the users' browser to link the consent record to them.