It's not because crowd scenes require technical skill to handle perspective, foreshortening and complex interaction.
Gluyas Williams was famous in the 1920s and 30s for his clever drawings of large groups.
And it's not because crowd scenes require the creativity to come up with a wide variety of faces and psychological relationships.
The great Albert Dorne was famous for his crowd scenes. Note how he handled the complex architecture of this mob.
No, what I like most about artists who specialize in drawing crowds is their obvious pleasure in the act of drawing.
Most artists working under a deadline look for shortcuts. They do a good job, but they want to complete a picture as efficiently as possible and get paid. But some artists just seem to love making marks on paper, and they regularly create unnecessarily grand challenges for themselves, like these ambitious crowd scenes.
In this category, I know of no better artist than the brilliant Mort Drucker.
This panel from the MAD Magazine spoof of Beverly Hills Cop is a good measure of Drucker's talent:
Despite the effort that went into this crowd scene, the drawing never looks labored.
Drucker, like Dorne, Williams and other artists in this rare species, draw with great abundance and generosity. You never get the feeling they are measuring their level of effort against the pay they are receiving for the picture. These are artists who love to draw, and it shows.