The key word here is 'security.'

Ad networks have been used to spread malware before, for example.

The data aspect does allow for much more nuanced pictures of people's lives than you might think - Google the story about the teenage girl who Target knew was pregnant before she'd told her family as a sample (and that was only from purchase data).

But realistically advertisers know a lot less than you think they do. They have the data, but don't know what to do with it outside of aggregate approaches that move the needle for a subset of their audience (who tend to just be very suggestible in general).