They've almost certainly considered doing that but I suspect it's a legal thing. Saying "Trump is a rapist" can be seen as claiming that "Trump was convicted of rape" which is not true so it gives them space to sue over a knowingly false defamatory statement (whether he'd win or not, it would be expensive and might halt the ads while it was being litigated)
Saying "Trump was found liable in a civil sexual assault case" doesn't have as snappy a ring to it and leaves Republicans saying bullshit like "well if he was really a rapist he'd be in jail/it's just corrupt civil court judges trying to make him look bad."
But saying "look at this silly footage showing that Trump is a numpty. What a silly crazy clown man" is depressingly more effective at making swing voters not want to vote for him. "Trump is evil" works for people who know he's evil but "Trump is a fool" works better for people who are willing to believe that the "evil" stuff might be overblown lies from his opponents' smear campaigns.
There absolutely isn't a good case and he'd probably lose because he's a rapist, but there's potentially enough wiggle room there that such a lawsuit might not get thrown out immediately which is potentially expensive and could get ads taken down while it proceeds.
I could be wrong, maybe they do run ads based on the rape but they might not think it's worth the risk for the reward if ridicule is more effective in their research.