Are they actually as simply to deploy as Excel? My guess would be that most streamlit apps never make it further than the computer they're written on.

If you have the right tooling (e.g. Streamlit.io) then yes, literally.

To 'deploy' an Excel file I go to Office365 and create my excel file and hit share

To 'deploy' a Streamlit file I go to Streamlit and write my single file python code (can be a one liner) and hit share