The problem is that 3rd parties are doing the packaging both on Snap and Flatpak whereas if they had followed proper security practice ONLY THE REAL DEV should ever be allowed to package their app as a Flatpak or Snap.
Says who? If it were the case, Linux would either be a nightmare of fragmentation or become centralised on one distribution. Distros need to be able to package their own software, and these are kind of like distributions. Also since we’re talking about proprietary software here, is it really any better security practice if the “real dev” packages it or somebody else, they both could contain malicious code.