Muscles are heavy. Moving weight takes energy. In endurance sports like running and cycling, carrying around muscles that aren’t contributing to moving forward is just extra effort for no more result.
I wouldn’t go as far as to call it anorexia, but upper body strength isn’t a big contributor, so while im sure that there’s a lot of conditioning involved; since having a good core helps your form, both in running and on the bike, you only want as much as you need, not more. Endurance athletes like pro cyclists and elite marathoners tend to be pretty lean. Triathletes, due to the swim, tend to be a little more evenly distributed, but they’re usually pretty slim too.
Compare them to Olympic (pool) swimmers, track cyclists, or track and field sprinters, who, due to the shorter duration and higher intensity of their competitions, require a heavier musculature than their long-distance counterparts.

For most of us, though, especially if we’re not pros or highly competitive amateurs, benefit from having a more balanced distribution,
If cycling is your main sport / fitness activity though, you’re still going to end up biased towards the legs.

They. Don’t. Care.