Many users would pay to have a subjectively better user experience. Sync was originally made for Reddit and had all that development time before Rexxit happened, and therefore, even after switching over to Lemmy, Sync is a more polished experience for many people compared to the FOSS alternatives.

I use Liftoff and Connect myself, and have never tried Sync. However, when Boost becomes available I'd love to try that out as well, as that was my Reddit app of choice.