Because hopefully it would deter folks from making a ressie that they know they probably won't make it to if they were going to be charged, say, $20 for bailing on it or not canceling with a good bit of notice.
CRT, Akershus, HDDR are all perfect examples. Would a family who has already paid in full for or paid a hefty deposit in advance for one of those meals make an ADR for another meal, at another location, at generally the same time because they're just not sure where they're going to be or know what they might feel like eating? I don't think so. I know even for myself, those meals that I've had to secure with a CC deposit or paid in full in advance were the meals that were "set in stone" and were meals that weren't in the "cancel-able" category just for my own planning.
Requiring a CC for all ADRs (just a deposit type of thing, like a lot of restaurants already do) I think would deter a lot of double-bookings due to people who can't make up their minds where they want to eat.
And don't forget V&A's ... they already do this.
FWIW, I also think shortening the ADR window would also help. 180 days is outrageous, IMO, and causes a lot of people to fret, stress and feel the need to book meals when they might not be ready to get down and dirty with "planning."
OP, I think it's rotten the way the CM spoke to you. Had it been me, I'd probably have taken my vacation dollars elsewhere. Disney has created this inflated hype over ADRs and its food. If a WDW employee spoke to me like that on the phone before I even set foot on WDW property, threatening to take the "magic" out of my vacation over something like MEALS, I'd be livid. I actually am just hearing about what happened to you.