My theory: the person in charge of a landing page doesn't want people bouncing off the site from the page they control (don't want the metrics to look bad!), so

My theory: the person in charge of a landing page doesn't want people bouncing off the site from the page they control (don't want the metrics to look bad!), so they tend to make it opaque to encourage click throughs. It doesn't matter what the best user experience is because they don't get measured on that. They get measured on bounce rate.