Ask HN: What's rationale for air miles being based on miles rather than dollars?

Why weren't frequent flyer rewards based on dollars spent right from the beginning? For decades an economy class passenger who spent $400 and a first class passenger who spent $10,000 on the same long-distance flight both got the same 5000 mile award.

I benefited from this greatly in the 1990's and 2000's because I always bought cheap seats. (I realize that most airlines have now corrected this anomaly, but only in the last couple years.)

It seems like common sense that you'd want to reward big spenders, not the vacation travelers. Casinos reward customers according to their spending and not for how long they stay.

So why did airlines make a strange decision to reward "distance"?