I get it, and your point is well taken.
I'm all for people following their passions, but art and running isn't an applicable comparison. If the only similarity is that neither are paid well and people still do it, then why not add working at McDonald's if that's your passion, or any job for that matter.
Art isn't objectively measurable by your peers, unless it's by the dollar value that your pieces go for, to which most people would agree that your name means more than the work you put out. There's tons of artists, who given the right exposure, would be more successful. That goes for actors, musicians, and a few other craft workers as well. And how many career ending injuries does a master sculpture have to worry about, potentially ruining his funding for the year.
Can't you see how that's completely the opposite of running? Running is 100% objectively measured. There's a race, you run it, your place is your place and your time is your time. That's it. So whats your motivation to do it, to see how fast you can get? The sacrifice for these guys is to negatively lopsided.
I'm not suggesting people don't go for it, I'm simply pointing out that there isn't enough motivation to boost the number of people going for it, and thus there isn't enough competition to keep us moving forward at the same pace (pun intended) as the East Africans, who have a TON of motivation, simply because 20k in Kenya goes so much further.
Rojo and Wejo keep blabbing about how much money you win for a marathon, or the 100k for the Boston trio of events. So, if you win a Major or this one set of races, you can maybe earn what a computer programmer in New York makes? Basically, if your one of maybe 15 guys in the world, you're going to struggle.
Imagine if you had a league of running where guys minimum salary was 150k... think of how many more people would still be at trying to make it, and how much more motivation there would be for elites to stay on top.
The better question would be, Why be a professional runner? It sure isn't the stability.