Releasing a game in Early Access means the studio can get some new funding (in case they were running out) and/or get early community feedback to improve the game, when it's still in a stage where it's easier to shape it. On the flip side, you do expose your customer to an incomplete and sometimes buggy game, which can diminish the player's experience. Not only during EA, but also when it's finally released. That's why studios might not like the idea thereof, but they don't always have a choice. Lastly, I should mention that it's also a risk to the customer because you're not sure what you're getting, if the game is released at all.