Doorway pages aren’t necessarily bad on their own. The issue is when they pretend to be the actual page with the info you're looking for, but they don't deliver. In a way, search results are kind of like doorway pages too—they just show you where the real info is. The problem started when people began making their own doorway pages, and it ended up being a terrible experience for Google users.
Imagine you're searching for something, find a Google result that looks like it has the answer, and click on it. The page loads, but instead of the info, it says, "Click this button to see the answer." So, you click it, and then it asks you to confirm again. You finally get to the page, but there’s still no answer—just a bunch of text and another link that says, "Click here for the best answer." You follow that, and it takes you to a book page, saying the info is in the book, and you need to buy it. So they trick you to visit 10 different pages and ultimately asking you to buy something from them.
A door inside a door inside another door; the endless rabbit hole! People used to make websites like this, and it was the most annoying thing ever.