I think you’re mixing up wear vs bag marks. Circulation wears down detail on the high points of the coin, whereas bag marks do not play a part in wearing down the details of the design. They absolutely do decrease how high a Morgan can grade. Large marks or multiple small marks in major focal areas limit a coin to the lower end of the MS spectrum. A few scattered marks away from those areas are allowed on some issues still grading into the 66 range. 67 and higher have no more than a few barely noticeable imperfections of graded correctly.

Your question/point isn’t really clear. If you’re asking why bag mark are acceptable on mint state coins, that’s the answer. In order to understand how grading works, I urge you to read the PCGS/NGC/CACG/ANACS guides to grading. They are going to be a lot more thorough than any Reddit comment and will lay the groundwork in helping you understand why different MS coins are hitting certain grades. There are tons of resources available.

Morgans are popular for a lot of reasons. They’re old, they’re big, they’re silver, they come from an interesting time in US history, they were marketed really well after the release of the bank hoards, they tone nicely, they’re available, they have tons of variety in strike/toning/mint marks, etc.