I suppose that in my case it's just a bit of laziness.
I rather enjoy just loading ten of those thirty round magazines on the workbench and not having to contend with loose rounds at the range or in the field.
And I enjoy camping in the mountains and being able to engage multiple "attacking pine cones"
without having to reload.
But mostly, I just enjoy using Stoner's truly superb AR platform. Both my 5.56MM and 7.62 MM rifles are among my all time favorites.
(I still enjoy my Garands, 1903A3 and SMLEs as well)