Additionally, due to the way that the allergic sensitization process happens - many people only develop a reaction after frequent, long-term exposure. So it's very possible to use a metal laptop without experiencing an issue for a long period of time, and then to develop problems down the road. This, along with the allergens sometimes being from contaminants in the metals (so not listed and somewhat variable) - makes it very hard to predict and to identify afterward.
So while I'm "rare", in that I know I've got an allergy with a very noticeable reaction that can be triggered by laptops - many people don't know that they are at risk of developing it, even though they are. Or maybe they know they have a nickel sensitivity (this issue is very common with people who wear cheap jewelry) - but they don't know a laptop can trigger it.
Unless there's a big push to do some sort of widespread contaminant testing on laptops, I imagine this issue will grow if metal-body laptops continue to be the norm.
Not everyone has skin allergies, but if you are mass producing devices that will be in close contact with skin for long durations, made with metals that are well known to cause skin irritation - sooner or later you're going to run into people that are affected.