I wish people would stop saying this. Sea of Thieves is Rare's most successful game of all time, and one of Microsoft's biggest releases over the last 10 years. They wish all their studios could put out hits like that.
The problem is that Rare has been a Microsoft Studio since 2002 and has only really made 2 "good" games (Sea of Thieves and Viva Pinata) and a whole bunch of shovelware garbage. Prior to 2002 Rare was one of the greatest video game developers to ever exist so to take them over 23 years and only get one middling/profitable game is wild.
Rare (and other game studios) don't just sit on a shelf. They employ hundreds of people, hundred of various expensive computers/machines/software/licenses, office space, marketing budgets, business expenses and so on. Rare likely cost Microsoft $30,000,000/year, every year, for 23 years or $690,000,000. This is outside of the existing $375,000,000 that Microsoft paid to acquire them in the first place - putting them on the hook for more than
a billion dollars so far.
I'm sure Sea of Thieves is a success, but there's no chance (doubly so with it being on Gamepass) that the
profit is anything near enough to fill the cavern of a billion dollars of mismanagement. Given the cost of R&D development, the continuous development, the cost of being on other platforms, the cost of being on gamepass, the hardware cost, and so on - it probably made 50 million or more, which is great until you factor it all in.
They also never successfully capitalized on any of Rare's IPs (Perfect Dark, Killer Instinct, Kameo, Battletoads, Banjo, Conker, etc) and instead severely diminished their value.
TL;DR - Microsoft needs their studios to be profitable more than 1 time in 20 years, doubly so if the studio is a legendary studio that was dropping bangers year after year prior to acquisition.