The actual question is why so many games in beta are released as a finished product.
The short truth here is that making games is actually extremely time consuming. If you go out of beta, that basically means that your game is 100% feature complete as of your initial draft. Many games in early access have quite a lot of features that still need to be implemented while the core game might actually work just fine.
Beta is nowadays a completely arbitrary label for a work in progress. At least when it's communicated to the consumer. Among the programmers it might mean something completely different depending on their definition.
In the time of online games there is no clear distinction between beta and a finished product anymore. Take Rocket League as an example. It released 2 years ago as a finished product. It was absolutely functional. Over time it gained so many new features that it's basically unrecognizable from when it released. The core gameplay stayed absolutely the same but it gained so much features and content around it. We're now almost 2.5 years in and it's still even missing the biggest and most requested feature since release, cross platform parties. Technically you could say RL has been in beta for over 3 years now if you include the fully functional public alpha and beta. The only difference to a game in beta is that the publisher said it's finished, despite obviously being not at all.
tl;dr The term "beta" or "early access" or "finished game" mean absolutely nothing today. You should always take a game in the state it is currently in and decide if the price tag is worth it. Just think of it this way; as soon as there is a price tag it should be considered a finished product that the producer deems worthy to sell and it should be judged on that merit.
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