Why are "point-to-point" courses not record eligible?
well, er, actually, they are not.
at least, not just because they are a point-to-point course.
the rule (IAAF Rule 260.28) says that the start and finish cannot be separated by more than 50% of the race distance. AND the elevation change must not be more than 1 metre per kilometre. in other words, the finish of a marathon cannot be more than 13.1 miles from the start, and, it mustn't be more than 42 metres higher or lower than the start.
obviously, point-to-point races are more likely to fit that description than loops, but it doesn't mean that any point-to-point race is not record eligible.
IAAF Rule 260.28
For a performance to be eligible for ratification as a world record:
d. The start and finish points on the course, measured along a straight line between them, shall not be further apart than 50% of the race distance.
e. The decrease in elevation between start and finish shall not exceed an average of 1m per km.