Probably because Game Pass has certain games the same day they release, that's about the only major difference I can think of.
I mean that would be an awesome bullet point....if they were games the caliber of what Sony and Nintendo produce. So far GamePass has seen what? Sea of Thieves, State of Decay 2, Crackdown 3 Day and Date. Games that didn't perform well review or perception wise. Sea of Thieves from what I understand could make a comeback tho.
Did it get Forza? Will it get Gear of War 5? Will it get Halo Infinite? I question how far they will take the "1st Party Games Day One" rule. Doing so could really cripple some well performing title's sales at retail.
It reminds me of when Sony first started doing Free games with Plus subscription. It was all fun and games until when the PS4 came out they saw a drop in lesser AA title sales because everyone was like "Oh, it will end up on Plus for free so I won't buy it". I think Sony facetiously didn't put many games as Plus titles (Like Knack for the longest) just to break that mindset.
PSN:Forevercloud (looking for Soul Sacrifice Partners!!!)