Reason 1: Too much mental overhead for too many use cases
People try to make dashboards as extensible as possible, but dashboards meant to answer many different questions are unwieldy. Answering your initial question almost requires a user guide for the tool, and dashboard navigation takes the mental effort best used for just answering the question you have.
Reason 2: Duplicated metrics across dashboards makes dashboards unsupportable
Creating simple dashboards to answer a limited set of questions helps to relieve the mental overhead of dashboard navigation. Inevitably though multiple dashboards often end up showing data about the same metric.
Let's say that metric is "active users". Maybe Dashboard 1 answers questions about "active users with the most spend" while Dashboard 2 answers questions about "when active users tend to become inactive". An updated definition for "active user" creates a maintenance problem, and you need to check multiple dashboards to make sure they still work! This becomes a huge task for a data team to make sure dashboards are still giving correct answers for business stakeholders.
Reason 3: Mental organization of dashboard creators ≠ mental organization of business stakeholders
Making a dashboard is a creative process. If it's done correctly, the dashboard can be very powerful. However, how one person designs a data representations doesn't always match how another person wants to view the data. This is dealt with by 1) having multiple "pre-dashboard" meetings before the dashboard is even published to make sure every user is on the same page, or 2) by forcing the end user to learn the dashboard creator's particular way of viewing data. Neither of these outcomes is particularly pleasant for the dashboard creators or the end users.
At SimplyPut, we think dashboards absolutely have their place (we've made many ourselves). However, many times users simply asking their own questions and getting answers is better than creating dashboards. See how we're thinking about empowering business stakeholders to answer their own questions by requesting a demo above.