Sadly, with games needing to be installed before you play them on most new consoles (which can take upwards of 20-60 min), we cannot test any games at the store. Also, most games require a day 1 update which can also take 15-20 min more with a good internet connection. However, our internet is very slow and basic just so we can test consoles that are traded in. Employees look at discs as they come in for trades. If they look too scratched, we charge for a refurb fee and that game disc is sent to the warehouse to be tested, checked, and/or buffed if it has problems that can be fixed.
Of course labor cost is significant and so is the cost to bring in new customers. I doubt it would be hard or expensive to have machines that at least test discs for data integrity.
We also do not have the counter space to have an Xbox One, PS4, Xbox 360, PS3, Wii, Wii U, 3DS, Switch, or a PS Vita plugged in or hooked up. This is why preowned games come with a 30 day guarantee, or can be extended by a 1 year warranty which is only $1-3 depending on the price of the game.
I know it sucks to bring a game home, have it not work, and have to come back to the store for a replacement or a different game if we have no replacement. But that is how most stores that sell use products work as we literally have hundreds of games traded in every day. It would take hours to test 100 games spanning 9-10 different consoles. But again, this is why they they have a guarantee for issues like this.
Since Xbox One and PS4 games install first, it's very rare for a game on those systems to not work since after the install, the disc is only needed for the system to check that the game is present, then it all runs from the Hard drive after that. I hope I helped with some clarifications.