Note: I am not defending theft in any way, shape or form, I merely think there are some deeper stuff going on here.
Here's my theory on this.
First of, it is important to note that Pokemon is a collecting game. Gota Catch Em All. Completing the Pokedex, finally having that rare Emolga you've searched for for ages or getting a Shiny Emolga EX card can be immensely satisfying, it is the reason we play the games. Here is my point. Some people don't care how they acquire these goals or can't acquire them by legal means (alternatively is to lazy to get them by normal means). Both are essentially the same thing, and yield the same result.
This is also very easy to perform over the internet, as you won't face any other consequence than that of your own conscience (if you have any). Stealing someone's Venusaur card in real life is a lot more dangerous, as the getting caught could yield actual punishment. The internet, however, grants near perfect anonymity, and since both parties technically have to agree to a trade for it to be performed, the thief can become more inclined to steal.
As I mentioned earlier, catching and having a rare Pokemon can be a awesome feeling. Finally having all your hard work pay off. Some people can get the same or similar feeling from stealing it. To successfully scam someone, to be able to deceive on any level, as well as getting that Pokemon to complete your Pokedex. To be able to take pride in a successful theft can yield the same emotional result as actually working hard for something.
Now, why do people think this is outright funny? Theft is frowned upon in all cultures, and most of us learn that from a very early age that it is both morally, ethically and legally wrong. My theory, is that joking about it, denying it to be theft or making the incident smaller or insignificant is a way to cope with the act itself. I highly doubt the thief outright thinking to itself that it is performing a theft, or that it's thinking "it's just a game, who cares?" By disregarding the victims emotions and bagatellelizing the event, it's becomes easier for the perpetrator to feel good and feel the 'good' effects of the theft.
So the combination of not being able to require the Pokemon by legal means or being too lazy to acquire it, combined with the lack of punishment over the internet as well as the thrill of theft and the option to joke it away lead people to steal, both in general and in Pokemon. Is theft a despicable act? Of course, and I feel for you and anyone else who had their hard work stolen. This is not a defence of thieves, but merely a insight to why theft occurs.