My question about client hints

At today's session I asked the following question:

Given that you can only send client hints if the site could run JS and set
a cookie, then
why can't the site just run JS on first contact, extract this data, and
store it in a cookie,
at which point it is available on all previous contacts.

As far as I can tell, the only difference here is on first contact. What am
I missing.

-Ekr