I do not understand this either, but officially they state:
"Our annual subscription model ensures that our product for Android, Windows and Web benefits from a flexible infrastructure, offering you advanced benefits like Cross-Platform library sync while enabling us to keep improving the product. We’re always exploring new pricing models to better serve our customers."
Which can be found here: https://support.goodnotes.com/hc/en-us/articles/10359438511119-Goodnotes-for-Android-Windows-and-Web-Subscription-FAQs#h_01H9FVJ66ZDF4YM0MZKVR86RH8

However I'm displeased with a lack of options in this regard and their explanation. If it is due to the cross-platform support, than I would like an additional option to just buy a one time license for 2 options — Windows and Android, since the website has to be hosted by them, so it's understandable, that it can't be permanently owned.