Alpha Tests are only up for a few days to a month at a time because the game is in the early stages of development, it is by NO MEANS a fully playable game. It will be glitchy, there will be very limited to none Quality of Life aspects of the game (Like in the last three tests, we literally had to run everywhere taking sometimes up to an hour of boring running to get to a location), and only specific aspects of the game will be available. Alphas are for us to test the first implementations of what the devs stick in for "testing". Then we can leave feedback on what we think of the process as well as what errors we found in their coding. This helps to improve the game quality at "RELEASE" time. In a nutshell, Alpha is EXTREMELY buggy and VERY LIMITED, Beta's are LESS BUGGY and HAVE MORE CONTENT. So the reason why it is only a month, is cause that's all the infrastructure that is currently set into place, its covers more than enough time to finish the content (which is usually a day or two at most), and allows us enough time to test it, for the devs to make patches, and us to test it some more.
What do we get from having bought the packages? We get to help "develop" the game in a sense. The devs use our input to determine how the game plays in many aspects. We get bonus "cosmetic" things such as titles, skins not available at release, extra characters to use, and experience to start us ahead of the rest upon beta and release time. However, the main reason for us to buy the packages is to help fund the developers to bring about the release of the game, and live through all the lovely stress that comes from anticipating the next alpha test phase of the game