You can kind of get an idea how it's complicated in especially in areas like this:



If you run Edit > Separate on that first and then do a shell of all the individual separated pieces, you'll see that when each piece is individually thickened, the "side walls" between the original and the offset are not really aligned with each other in any good way:




That's the part that's pretty different with doing a thin-wall shell of a solid instead of an open surface, with a solid the "side walls" come from surfaces of the original solid rather than needing to be totally cooked up.

- Michael