As PPs have said, children's meals are usually loss-leaders, or at least profit-neutral.
Restaurants don't want adults not to come to their business on account of it being too expensive to have to pay for the kids as well - especially as a lot of adults will then have a glass or two of wine, coffees etc. that carry a much higher profit margin.
Many children are also fussy or ambivalent to the treat of a meal out, so parents are also very reluctant to pay a lot for a plate that may just be pushed around and left anyway. That's maybe one reason why many places say no children's meals for children over 12: because they not only eat a lot more by that age, but are usually less picky and value the experience of a meal out more.
From a restaurant's pov, an adult wanting to buy a child's meal for themselves, as they have a small appetite, is a bit like an adult wanting to pay half price on the bus, because they're only 4'10" and a size 6.
I wonder if this is partly a symptom of people frequenting restaurants more routinely than they used to. If a meal out is a special treat, a lot of people will 'save themselves' and only eat a very small amount beforehand, so they can push the boat out and/or ask for a doggy bag at the end; whereas if you eat out on a regular basis, you would probably only be eating the same amount as you would at home and not see it as special.
Loving the plastic frog plate! I can see how mortifying it would be as an adult to have your meal brought on one of those, with some crayons and a Billy Bear colouring activity sheet 
I wonder if this is partly deliberate, actually, to deter adults from ordering kids' meals, if they know they'll be brought out in an embarrassing way.