Phone UI paradigm isn't as easy as "Let's just scale down the Desktop/Tablet applications". In case of Sailfish OS development history, migration from clunky, but closer to Desktop, Hildon framework of Maemo 5 to purpose-built Qt applications of Maemo/MeeGo Harmattan was obvious quality boost. Same with webOS and UBports: app ecosystem starting from scratch within their corresponding "DE"s. If convergence-style cross-device applications were somewhat easily achievable, that would've happened in the early 2010s (Windows Phone / Windows RT? Ubuntu Touch getting more attention from investors?). It's the same story why Android Tablets suck and iPadOS doesn't, but in other direction.
And/or, it's a simple matter of time/money being spent on streamlining the experience. It's not like Sailfish OS is perfect (Qt6 migration is way overdue), but Jolla has already figured out lots of integration details which will become teething problems for Droidian and such. Including, but not limited to VoLTE support.