Just because a company has lots of money doesn’t mean they just pour all of it into a project.
I am a software engineer at a large/rich company so I have seen the process.
Here’s how a project goes:
They decide to make a game/product: they assign it a budget. The budget might have some flexibility but it’s not unlimited. The managers have to make the project work within the budget. The budget pays for Engineer salaries, outsourcing, and other resources.
Each engineer has a limited amount of time they can work in a day, and you must account for blocking issues, vacation, sick days. etc.
There is likely a very long list of bugs. For this game probably hundreds if not thousands!
Bugs are not a piece of cake to fix, they can take anywhere from a few hours to months! The more complex the software and the more unusual the bug the harder it is to fix. These games have millions of lines of code.
Most likely not enough engineers to fix all the bugs concurrently, so they have to prioritize. While they are working on some things, the rest go unfixed until the engineers are unblocked.
The company will not just pour unlimited money to hire hundreds of more engineers, this causes issues with being able to manage and train all these engineers and then needing to downsize the team after the game is more stable.
By the way as far as I can tell X-plane has fewer than 5 engineers working on the game engine. A studio like Asobo probably has a few dozen engineers and may be able to outsource some or borrow some manpower from MS, but most likely they have to manage it on their own. MS’ other engineers are busy working on other games and all the other software they have to manage and fix.
It’s not magic, it’s hard work, and it takes time!