Some get sealed some don't. Often there can can be and air leak through the spoke holes or valve stem and for whatever reason the sealant doesn't make it there.

Sealants have solids in them to plug the holes on the tire. Just a theory...But maybe the there is gap in the tape (from poor installation) that is just small enough that air escapes but the the solids in the sealant can't get through tape and to the spoke hole area to seal it up.

The sealant can also separate over time. The sealing stuff you want dries up and the rest seeps out. It's not a perfect system but works well.

When I used tubes, I noticed I would lose more air from tires than I do with tubeless tires.