I'm not sure I agree. Depending on how we define 'crash', I've certainly seen ads render pages unusable or prevent my access to content. Infinite request loops, script crashes that take down desired video content, improper loading covering content, and more.

The obvious example would be sites with mediocre mobile pages fighting to serve ads. It's pretty standard to hit broken redirects, script issues, and all sorts of other crash-like behaviors visiting those.