The thing is...games aren't cheap. They are $60 (now $70) regardless of how good or bad they are. So yeah...if I'm about to pay full price for a game day 1, you better believe anything below an 8 is trash.
If you think games aren't cheap to buy, you should appreciate that tens if not hundreds of millions in investment is a huge responsibility that needs to be carefully considered no matter how big the corporation funding it.
With sequels I assume most people would like continuity, in the sense that the same creative team gets to improve on what they did first time around. Now consider how that's impacted when "success" can only get judged years after release.
Games are made by people, and people need to get paid for their time and labour. So if you wait months for a sale, the people paying their wages will have had all that time to decide whether to put them to work on another, potentially profitable project, or to lay them off.
This is why launch window sales matter so much. There's a window of opportunity that doesn't last forever.