Holiday hams (for Thanksgiving, Christmas, New Year's Eve, and I think... Easter once?) have given me (and family members) food poisoning any time I have not cooked them. I don't know what my mom/dad/grandma/etc keep messing up or what they were buying, but it's happened probably 5 times now and it was absolutely the HAM. I will not eat the holiday ham anymore, thank you very much :blobangery:
("Why didn't you stop eating the ham earlier?" - because I was a dumb kid, ham was better than most of the food, and didn't know why I was throwing up but I still started putting 2 and 2 together as it kept happening I'll tell you huwhat :pepereeeee: :cryingl:)
So, you'd hire me to make a painting for you ...which takes extra time out of my normal schedule and I'd have to make something to your specifications rather than what I want to paint. You're paying for me to make something new rather than something that's completed and I don't have to worry about. So I would probably charge more than a normal painting (wouldn't you?).
But then you think that you should own the copyright to that image so you could, say, sell the image on prints/ shirts/ merch/ etc, and none of the money from those sales go back to the artist?
I think most artists would be fine with licensing the image to you (so they would either get a percentage of the sales) or allowing you to use the image for personal things or something you're likely to make no/ little money from - you can just ask if you want that to be included in the price (you may need to make up some documents if you plan to sell it and give a percentage, unless you're giving them a lump sum).
But... you can see how this could get out of control pretty easily if people were just allowed to make money off of someone else's images, right? I know the copyright laws are a little crazy (usually last too long, for one) and corporations can go overboard protecting their things - Disney will come down on you HARD if you sell their character art unlicensed, for example - but imagine the opposite scenario. Someone from Disney commissions a painting from you, then resells that image on their merchandise as a "Disney" product, making much more money because they're a big business. Even if they included your name (which they probably won't), very few people will actually find your work and buy from you (so the "your art is getting ***exposure***!" argument usually isn't applicable either). You're also not working for Disney and if you had known that you were, you would have asked for a much higher price to accommodate for the fact that they would make so much money off of it. Copyright can be annoying and downright bad sometimes, but it can and does protect the little guy from this sort of thing (sometimes).
And hey, "what's the point in commissions if they tend to cost a lot more" - are you *only* interested in the ability to reproduce the image and not the original piece? That's fine, as long as you make it clear that that's what you want. Companies that make trading cards (say, MTG cards?) are often only interested in the image and not the original, and pay for the ability to reproduce that image on the cards they sell. Not a problem - as long as it's clear.
TLDR: Commissions are for getting what *you* want the artist to paint/ draw/ make. If you want the copyright as well, ask for it?