Well, several reasons, but mostly because it all dips into spoilers and inaccurate reviews of the product. While we all want to seem like we don't care about what others think, a game's success is gauged by its review scores, and particularly metacritic is a very important one. If you didn't know, I can tell you in some cases the success of a game and the incentives the employees are paid are based in whether the game hits a certain metascore.
So I don't particularly care about how other experience it, but I do about how others reflect their experience of an unfinished product upon the internet, where their uninformed, anonymous opinion does count, even if a tiny bit. Even when people acknowledge that they are dealing with an unfinished product, truth is they don't, and they are quick to either praise or write off a product that in many cases will take an U-turn. There are countless instances of games that didn't look good at first, and turned into masterowrks that lost their momentum for releasing the unfinished product too early, and the other way around, too. I don't need to speak of, for example, No Man's Sky.
So that's why I wish products only reached the public in their final form, so they could be really and fairly judged, and also enjoyed collectively without spoilers, or the stain of others having an inaccurate opinion of it before it was actually a game.