Physical copies and Digital copies of a game are different, in that one is physical and one is digital. That's a really dumb sentence, but.. In essence, it works like this -

Physical copies will hardly drop below $20 - $30, and many games last for many months at the $60 price point. One large reason for this is because publishers need to build in the fact that the used game market exists, and one copy of their game could possibly be owned by four or five people.

Digital copies will start at $60 but, almost always, drop much quicker than physical copies. It isn't unheard of to find games from last year for $15 or less, and GOTY copies of the game with a bundled season pass or DLC can go as low as $5 sometimes after another year. A big part of the reason this can happen is because one digital sale will stick with one person - other than refunds or pirating or DRM-free, the digital copy won't really get passed from friend to friend.

Physical game - one sale for $60, six people play it. We could effectively call that sixty bucks split across six people, ten bucks each.

Digital game - six sales for $10.

This is a large part of the reason why digital games can drop in price way quicker than physical games. Publishers can bank on the fact that they'll get money for each sale, rather than one game getting bought for $60, given to a friend, sold to gamestop and re-sold for $20, back to Gamestop, sold again for $20, and then passed around three friends, where the publisher only gets the initial $60.

We don't want digital games to be treated like physical games, otherwise we can say goodbye to those insane $2.99 Steam deals that come around every once in a while. Pretending that physical copies and digital copies are the same thing and that we should get the right to share our libraries willy-nilly with tons of friends, or that we should get the right to sell our digital games back, means that we kill the cheap digital game market.