Why series are rated before saison finale ?

I can't understand why serie global note can be make before it's complete, if the all saison is rated by some after a trailer/review or even 1 to 3 ep viewed, can't be fair.

For exemple Willow 2022 is rated 5.2 but Ep are 6.4, 6.5, 6.4, 6.9, 6.1, and 7.4 so far, meaning 6.6 total.

So is the 5.2 fair ?