If it is impossible to make a living and work on these apps with either reasonable app pricing or no ads, then why is Sync the only app for Lemmy with these strings attached?

Sync is a "professional" app. The others are mostly hobbyists. That's the ultimate difference. Same as with Apollo. You had a bunch of open source reddit clients, but a hobby piece of software will seldom outshine a professional product. Sometimes it happens, but that's the exception not the norm. Sync has been incredibly stable for me over the last day. Memmy on my phone and Connect haven't been. No shade to the devs, but devs approach a solution differently when that is how they support themselves. Passion can only carry a project so far, speaking as a dev myself.