The common explanation is that Hershey's chocolate contains some amount of butyric acid which tastes/smells like vomit. Adam Ragusea on YT did a pretty solid video on it.

Why Hershey bars taste like vomit (and I love them)

Not all US chocolate tastes like Hershey.

American regular grocery store brands of chocolate are horrible. Hershey’s and all the rest. Of course it’s the cheapest ingredients drowned in sugar to cover the otherwise poor taste. Cadbury’s now tastes waxy to me. Used to be pretty good. All in the name of profit.

Aldi in the US actually has really decent chocolate, their Choceur label for milk chocolate is quite good. Good luck finding it, I think the tariffs killed the availability, haven’t seen any for weeks.

I thik a lot of the US stuff are things you have to grow with. If you taste them once you're an adult, you see them for what they are and they'll never work for you. Hershey and Oreo being the obvious examples for me.

Hersheys has always been horrible but we never had any choices when I was growing up. I’ve had a huge variety of tastes though, from bad to horrible: I can only assume one of the many problems is either lack of environmental controls or keeping it way too long. We have much better options now and I haven’t voluntarily had Hersheys in a couple decades.

However I object to Your criticism of Reese’s. It’s fair to say it’s not quality chocolate, fair to say the filling isn’t really peanut butter, fair to say it’s ungodly sweet, but chalky? Yes, I love it: why?