Because they are huge, dense, and you know and trust the code….but should you?
Malware distributers are starting to piggyback on these libraries to get their malicious code into sites. They can simply take the enitire script file, insert a few nasty functions, and host it on an official-looking domain.
The best defense – a script whitelist on your browser (like NoScript). Without it, you might be susceptible to an attack from one of these malicious scripts.