They still have bugs from beta unfixed. If they find a problem and change it, it could affect and break 50 more things. It's ongoing for everything they fix.
It's like a building. You build a house and decide you want to add another room. You go in an add it, but then you find out theres problem with the wiring. You then fix the wiring, but then you notice theirs a problem with the lights. You hope everything works fine the first time, but you always have the chance of breaking something and having to fix it. Which then could cause other problems.
So having a perfect game with updates and expansions, then they would have to do some big beta testing.
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