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

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.