I'm guessing it's so that there is no net elevation change, otherwise people would run down a mountain.

My knees hurt just thinking about that.

It might be about elevation, but of course it could go up the whole way too, Pikes peak. I would hazard to guess it is because of wind, even on a dead flat course with 0 elevation gain, you could have a giant tailwind the entire way, thus making records that are not accurate. Just like running on the track, get too much wind, and you cannot set a record(as long as it is behind you of course)

I get that elevation gain and tail wind would disqualify a record attempt, but simply saying a course can't be a record setting course simply because it's point to point baffles me. A point to point course, just like a loop course, can vary in elevation gain. What I don't understand is, since it's USTF certified, why they don't do it like a 100 m dash. Call it a non-record qualifying race just before the race starts or afterward, depending on wind conditions.

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My marathon PR is "under three, high twos. I had a two hour and fifty-something."

why they don't do it like a 100 m dash//

Maybe because it would be virtually impossible to measure the wind over 26.2 miles? On a track with a 100 meters in front of you, in a controlled arena is one thing, out on the road for 26 miles with twists and turns and varying winds all over the place, just not possible or particle. That is why they use loops or out and backs, generally speaking you will get equal amounts of head/side/tailwinds within the time frame of the race. Not always of course, but certainly takes most of what a point to point poses out of the equation...

There are two basic rules for "official" marathon courses.

1. The finish needs to be no further than 13.1 miles from the start. Point to point could work if it's a horseshoe or something like that.
2. The total drop on the course can't be more than 1m/1km, so basically pretty flat. This isn't an issue when the start and the finish are the same spot but when they are different it becomes and issue.

1) incorrect.. Start and finish must be within 1km of each other

IAAF Rule 260.28
b. The start and finish points of a course, measured along a theoretical straight line between them, shall not be further apart than 50% of the race distance.

If my math checks out, 50% of 26.2 is 13.1.

Are NY and Lonfon official courses?
They're definitely more than 1km between start and finish

Not valid for world records, if you want to dig deeper into the rules

Incorrect.