[Shouldn't the delivery time increase based on the gig quantity?]

Hi. I’m a colorist, I color line arts.  
Let’s say it takes me 8 hours to color one illustration, on and off.  
Let’s say the delivery time is 2 days.
I've noticed that when buyers order multiple illustrations (e.g. 10 at once), the delivery time stays the same. It’s impossible to color 10 illustrations in 2 days. Shouldn't the delivery time automatically increase based on the gig quantity? That would make more sense, in my opinion.
Chris