For example, compare the Reese's Peanut Butter Cups to Trader Joe's peanut butter cups [0]. It may be that the Reese's ones used to use better chocolate or it may be that my tastes changed as I grew up. But I used to love them as kids and now they taste off. Similarly for Twix etc.
[0] I'm sure other stores sell peanut butter cups too. There's nothing special about the Trader Joe's ones other than they are mass produced and use better chocolate that Reese's.
EDIT: Flavor change may be due to cost-saving measures like replacing cocoa butter with vegetable oil https://www.today.com/food/chocoholics-sour-new-hersheys-for...