As such they provide EXCELLENT opportunities for terrorist attacks.

Setitimer told you correctly. Pipelines are very easy to monitor.

That aside though, there are several ways for terrorists to attack using any mode of transport. That is hardly something exclusive to pipelines and can't be used as a justification against using them.

Pipelines also leak naturally over time. The length makes maintenance incredibly expensive, sometimes almost impossible in more isolated regions.

Lol, what? Pipelines do not naturally leak over time. Where the hell did you get that stupid idea? And the length of the pipeline has absolutely no bearing on how expensive maintenance is. You only have to work on a small section of the pipeline. Not the entire pipeline.

And seriously? They are impossible to get to? Name one area where they are that is impossible or even remotely close to impossible to get to.

It's not in the best interests of the pipeline owners to prevent environmental damage caused by poor maintenance either, it's cheaper to simply let them leak and ignore environmental consequences.

Hahaha, well good thing you don't work at any of the companies responsible for them because that is horrible advice. They care a great deal about leaks. First off, it's their freaking oil. You think they want to literally leak money out of the pipeline? Secondly, you don't get to just "ignore" environmental consequences. I don't know what kind of fantasy world you live in but polluting the surrounding area has very real monetary and PR costs. The government does actually fine you and you most likely will get sued by people who were or even weren't actually affected and your reputation will take a hit.

Oil companies hate pipeline leaks more than anyone. It's their oil being lost and they are the ones who bare the cost. Pipelines certainly have downsides but none of the ones you listed are real and even with the downsides pipelines are still by far the safest and cheapest method of transportation.