Well in the case of AFM they grossly under-produced that game. It's run was cut short to make room on the line for some crappy Pat Lawlor game that was over-produced and subsequently discounted to purge remaining inventory. It's a shame because AFM earned very well and sold well too, and they easily could have sold more of them (and this same scenario was repeated on MM but not nearly as bad, they made like twice as many MM's as they did AFMs). I think it is crazy if AFM is selling for 7k but they are actually fairly uncommon compared to other WPC era games in terms of machines produced.
MM had great demand right after it was released. It earned amazing, the ops loved the game as it had very long legs compared to nearly every other pin released in the second half of that decade. Collectors and players wanted them early on but the ops wouldn't sell them. Ops actually wanted additional machines but the company moved on to the next game and never looked back. They could have run a couple thousand more of them back then and moved them all. It is one of my favorite games of all time and I'm happy to have it, but there's no way I'd pay the going rate for one.
I had no idea playchoice games were increasing in value or were even at all desirable. You can get the exact same game on the NES. That machine was really like a big pay-to-play advertisement for the NES back when it was around. I would imagine some of the increase is just based on the nostalgia factor of that generation being adults now with some money and want to collect something they remember playing from the past. I've been collecting punk rock records since the early 1980s and it's crazy how much a lot of the records I bought back in high school sell for on eBay now. It is bound to happen with anything remotely collectible as the years pass.