Take Ubisoft for example. They had thousands of people working on AC3, and look at the piece of trash that it turned out to be. Now take FTL: Faster Than Light. They had 2 people working on it and it's better than AC3. I paid $50 for AC3 and played for 25 hours, and I paid $5 for FTL and played 50 hours.
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Money, because they need to pay publishers and parent corporations combined with this fun thing called Nostalgia, which has a nasty habit of stabbing you in the back.