Yeah this is a great question. There are 3 main reasons:

1) A lot of users don't have the time to learn Power BI or Tableau. There is a learning a curve and many people I speak with aren't familiar with the underlying data concepts that you need to use those systems.

2) Many companies don't allow their employees to use new software without getting approvals (especially if that software will be accessing sensitive data). But almost all companies do allow employees to use Excel.

3) Oftentimes the easiest way to share a dashboard without having to worry about training people or giving user permissions, is to simply share an Excel file.