It steals drivers attention where focus should be 100% on whats happening on the road and around it. Every % of attention stolen is a big win for advertisement, and increases chances of bad accident. Do a bit of statistics on number of drives per day globally, number of accidents, increase that chance even by 1% (which is very debatable but in many cases its much more), and you get nice number of number of deaths and damages done daily by ad business around the roads.
