Cost is the easy answer - as with any optimization, I guess. It can also be really annoying sometimes to scale a monolith - a big one can have a lot of differe

Cost is the easy answer - as with any optimization, I guess. It can also be really annoying sometimes to scale a monolith - a big one can have a lot of different parts like multiple threadpools/ database pools. One component hits a threshold, you add more instances, now your other component is overprovisioned, maybe even failing due to too many DB connections, or you hit weird thread contention issues. I've had some pain in the past scaling monoliths in similar situations.