The internet is supposed to be an information retrieval tool. Advertising’s whole goal is to stand between you and the information you actually want. And it does so by trying to anticipate instead of the thing you want, the thing you are most willing to buy next, whether that’s actual products with money or propaganda. Whereas an ad in a magazine about computers offers me relevant ads for products about computers. And if you read old ad copy a lot of it is a serious effort to try and convince you to buy their product. From some kind of argument for it. Instead of simply using statistics and data to predict what you will buy next. So this required the product to actually deliver something to justify the effort to advertise it.
Why? I don't see the difference between a webpage and the magazine here, except that I guess you're assuming the webpage must be showing an unrelated ad.