Because Porsche makes iconic sports cars and has done for over 50 years, most of which have been manuals, and most of which are still on the road. With such a rich heritage, not every decision will be based purely on ‘business’ or economic factors.
Just look how popular cars like the 911R, 991.2 GT3 with Touring Pack and GT4 have all been - with manual transmissions. And the 991.2 GT3 was offered with a manual option, after strong complaints from enthusiasts that the 991.1 GT3 was PDK-only.
May as well say ‘Why offer bright cheerful colours when 90% of buyers choose monochrome colours?”.