Those projects have real difficulties to retain enough developers even to keep lights on. And even higher-ups want, they can’t force allocate developers (in Google’s current power structure) other than providing some incentives including promotion.

The end result is a doom loop that if the team can’t retain enough staffing, developers will be overwhelmed by the boring maintenance work, which leads to more team transfers. They do use contractors to mitigate the issue but IMO it doesn’t work in every case.