Art theft is part of the reason I don't share much of my work (other than tests and experiments, like on my art thread here). Even putting stuff up on my website is daunting, and it really shouldn't be.
Yes, as others have stated, some may just have trouble coming up with designs or color schemes. Maybe the person is interested in the adopt's design. It's not always about the art or art style.
Also, buyouts of that range aren't outrageous (in my opinion). Take into consideration the time and effort the person took to make that adoptable. They had to come up with the design, the colors, and then draw it. Not only that, but buyouts may be the amount someone can offer if they don't want to go through the auctioning process to receive the adoptable! (Plus, if the buyout was super low, there's kind of not a need for an auction, would there?)
Also, yes, please don't steal art! Saving the image and keeping it, calling it your own, using it, etc. without you gaining proper ownership of said art would be considered art theft.
Hopefully these comments from others helped you understand this better! ^^
That's why watermarks exist. So no one saves a piece that does not belong to them. Art auctions exist so people can mainly share their work and make some profit too (nothing wrong with that). It makes them happy and those who bid on them happy as well.
Of course you could save the pic but the person who saved it doesn't have any rights or ownership over the piece/character.
And a lot of people but adoptables since it's hard to come up with stuff you like that on their own, some people have amazing design ideas, I love getting adoptables and custom designs when I can. The artists generally put a good amount of thought and effort into their designs too, I know I do, and while they may not be the best, I do try. Also watermarks are a thing so that people can't go around claiming a piece of art or character as their own. I didn't start putting water marks on my stuff until recently, and while pictures look better without it, it's for the safety and protection of the art.
aa adoptables can go for $100s easily, i've seen it frequently on deviantart [although those were vERY high quality + detailed designs]. people are willing to pay different prices for art + design, and most of the time artists undersell their time + effort big time just so people don't complain about the price. c: