If you're going to spend $69 billion you have to do your due diligence. You want to know what you're buying.

They knew what they were buying - a beloved studio that has churned out tons of "classics" over the past couple decades, which has been able to sell and resell one of those classics (Skyrim) for like half a dozen different consoles.

There are only 2 options: Either the Xbox division at Microsoft is completely incompetent or they knew they had a stinker and decided to sell it anyway.

There's actually a third option: Microsoft knew Starfield would sell, stinker or not, because Bethesda has a very loyal fanbase that will eat up anything they put out. It didn't matter to Microsoft whether the game was good or not, they knew they'd make their money either way.