Are you asking why a filter doesn't suffice, or why uBO doesn't automatically load the userscript?
The answer to the former is that the script swaps in a low-res replacement stream while the ad is running, which I don't think a filter can do. As to the latter, an extension automatically executing arbitrary remote userscripts supplied by third parties would be a nightmare for security.
The answer to the former is that the script swaps in a low-res replacement stream while the ad is running, which I don't think a filter can do. As to the latter, an extension automatically executing arbitrary remote userscripts supplied by third parties would be a nightmare for security.