Surely that's a big security risk with them being much easier to mark?
It makes them easier to mark, especially inadvertently, but that's not a security risk, because the marks are so obvious.
Cardback designs have two major purposes, outside of aesthetics/branding:
1. Obscure unintentional marks and signs of wear, so that the cards do not have to be replaced too frequently.
2. Differentiate the decks in use, so that sneaking an outside card into play is more difficult.
Using two different color decks at a table - and different colors at adjacent tables - helps a bit, but nothing is cheat-proof.
Also, full-bleed cardback designs make it easy to spot a boxed card in the deck; that's why many magicians prefer cards with a white edge around the back (people spotting a boxed card can spoil the trick.)