What insanity is it that possesses website-makers to think that it's OK to have locked-on-screen ads on the mobile versions of their sites when this isn't even

What insanity is it that possesses website-makers to think that it's OK to have locked-on-screen ads on the mobile versions of their sites when this isn't even common practice for desktop sites? Mobile devices don't have very much screen area for content to begin with and reducing it further in this way is not something we should accept.