It's a question of how closely the ID can be inspected for accuracy.
When I go to the store and buy beer, they want to see my license. They always make me take it out, which means the clerk can feel it, and the "feel" will often give away a fake to someone who has handled thousands of legit IDs. Next, they look at the pic on the ID and then look at me to make sure they at least kinda match. Nobody is really holding them up for a side-by-side, but you at least kind of have to look like the guy on the ID, which immediately limits the pool of people who could be faking my identity. Then, assuming it feels right and looks like me, they scan the ID and verify a record of that ID card with the state, which simply confirms that the state issued such an ID.
Between those three things, the system is actually pretty secure. But when you ask someone to scan in and email a copy of the front of their license as proof of ID, all three of those "checks" are eliminated.