Early Access games are sold and otherwise made accessible in various stages of their development, but they are not "finished" games. Metacritic chooses to apply a final score to a review only when it is a review of a finished game. Therefore, while you might see early access reviews for these games, they are not scored, and those reviews do not contribute the eventual Metascore. Many critics will provide a "provisional" or "early access" score which we will append to their quotes, but, again, those do not contribute to the eventual Metascore which is generated upon the game's final release.
Similarly, Metacritic does not enable user ratings and reviews until a game is fully released, beyond its Early Access stage.