Why don't ISPs peer with everyone?

Hello, I wouldn't consider myself a network engineer, nor do I have any formal training, but why don't ISPs peer with every other ISP? It would only save EVERYONE money if they did this, no? Only issue I see is with possibly hijacked / malicious AS owners, but that's not very common to do without being caught. All the whole "don't peer with this guy" only makes your customers have worse latencies and paths to other people, making the Internet less healthy. Thanks, Rucas PS: sorry if I sent this twice; client lagged a bit.