Probably for the same reason my hammer isn't very good at screwing in screws, and why my screwdriver isn't very good at taking me across town, and why my bicycle isn't very good at hammering in nails.

Because Microsoft tried that and (many) people hated it.

Monoliths tend to suck, especially when bureaucracy kills great ideas. And then people implement great ideas on their own.

Now, since they tend to be unconnected, one can go back to the "mother ship" or innovate a system of inteconnects or API's to get them to work together.

"In suckage, there's opportunity."

In fact Linux and open source in general grew from dissatisfaction with monoliths, and compare the good they've done compared to the dinosaurs in the computer history museum.

Many companies gravitate towards monoliths as growth opportunities, and IT managers like "one throat to choke" (heard that WAY too many times. ) but flowers, people, and apps are different for good reason.

Because one instance of any given tool cannot serve everyone equally well. What's great for you may be terrible for me and vice versa. Combining everything into a monolith necessarily involves heavy and arbitrary compromises to the usefulness of the tools.

Keeping the tools separate means that you and I can select the specific tools for each thing that suit us the best. It allows optimization.