Earlier quoted context omitted.
Because they can charge $40-$80 (or more) for a fluoride treatment. It's actually how they make money on cleanings. If you don't opt for the fluoride treatment it's a wash.
And fluoride treatments likely don't do much. The main benefit of fluoride is from ingestion by children, while their teeth are forming.
Here's a meta analysis for you: https://www.ncbi.nlm.nih.gov/books/NBK401516/