For many reasons including:
There aren't many places that do not have Office installed. People are very familiar with it and even if they aren't, they can usually make their way around it in with only a little coaching.
Most companies use Excel for basic analysis, charting, and data delivery to non-technical report recipients.
Deliver a raw dataset and allow the end-user to pivot, chart, etc. It takes the strain off the analytics team for basic tasks and gives the end user the power to do what they want with the data. Tableau seat licenses aren't realistic for most companies and building some sort of analytics platform from scratch in your language of choice isn't always something you can do in the short term and provide more functionality with less training required.
I could go on, but that gives you a general idea.