They take up more conscious space. You have to parse them more carefully if they are right in the middle of the content that you actually want. Those big blocks of in your face adverts don't require any attention at all and they it's trivial to use your brains inbuilt adblock to filter them out.

So, if one must have adverts, the usual form factor is vastly preferred. Especially if the alternative is trying to masquerade spam as legit content.