People tend to migrate to the "fast" lanes (the left), so you naturally end up with people either passing on the right or tailgating them. Of course, you should always be in the right lanes except for passing.
I do wonder if let's say there's someone in lane 3 of 4 going slower than the limit and you can pass them safely on the right in lane 1 that it's probably safer to just do that (assuming you're not flying along!) than moving over 3 lanes to the left, passing then moving 3 over back to the right.
First, I should note that I 100% agree.
However, as the highways have become more crowded, it has become less practical to actually adhere to this.
If I'm going 70MPH (the prevailing speed on the NYS highways where the legal speed limit is 65MPH tends to be somewhere between 70 and 78MPH), and there are a half-dozen cars going 60MPH in the right lane, with just barely enough room between them to safely get in (ie, there's sufficient stopping room between the rear car and me, and between me and the front car), then technically I could pass the first car, pull back into the right lane, pull almost immediately back into the left lane to pass the next car, and repeat several more times. But that's not merely tedious, it's dangerous.
It's even worse when instead of a half-dozen regular cars going 60MPH, it's three tractor-trailers. Particularly since "safe distance" for them is much longer due to visibility issues.
...And, of course, sometimes when I'm doing this, some yahoo going 80MPH will zip up behind me, pass me on the right (completely ignoring the aforementioned safe distances) and then zip back into the left lane to careen onwards.
I used to adhere to this but it does not work in practice.
Right lanes have the highest churn-- people getting off, people getting on. If you're not planning to do either, you have no business being on that side of the road.
This idea about left lanes only being for passing only ever works when you're overtaking a tractor doing 15mph on a country road. The concept does not scale to freeway speeds or traffic. You're not legally passing anyone already doing 70.
Rather, it's idiots weaving through traffic. They treat the flowing traffic as basically static obstacles and things can go very wrong if they cease to be static.
I've also seen it because the guy who flew by me on the right was impatient with the amount of clearance I left before moving back to the right lane--cut right as soon as there was a gap wide enough to move through.
