I think it's is sort of like why does an open can of paint dry out even though the top layer skins over and dries. Simply put, I think the air migrates through both the tires and sealant. It does it slowly, but clearly does it faster than it does through tubes. Which of course also slowly go flat, though usually very slowly.

I've been riding about 6 years, 25,000+ miles, mostly road riding. All road riding with tubeless, never once had to spend any time on the side of the road dealing with a punctured tube. Maybe had to plug a tire 4-6 times total. That takes less than a minute to plug, then time to reinflate as needed. And I haven't even had to do that in close to 2 years. I live in the desert southwest. Lots of goat head and other things.

No question tubeless takes some time to setup and maintain. It has a learning curve, as does using tubes. For example, many examples on BF of people having mystery leaks with tubes likely due to damaging the tube during tire installation. People learn to avoid that. Just as people learn to tape a rim properly.

Even if tubeless were 10X harder to deal with than tubes (and it's no where near that involved), I'd still take 10X more effort in the comfort of my shop over dealing with replacing a tube on the side of the road. And I'm not saying that dealing with a tube on the side of the road is some horrendous thing. From comments I've seen here on BF, many seem to have it down pat and deal with it quickly. And those same people likely prefer that over dealing with tubeless. Different strokes for different folks.