Pretty much the same as people who would vandalize another persons property in general. However I see those that come across the internet as somewhat avoidable. Avoiding a site that requires additional downloads to get the file you want. Paying attention to what the extension of the file you downloaded. Anything with the .exe extension is bad news, unless you were actually downloading an .exe. Adbockers and antivirus/malware are your best friends.
With caution the threat of malware is, in my opinion a little more avoidable than that random asshole that decided your car needed an egg wash, or that your house was in serious need of increased air flow through broken windows. A new coat of paint via the cheapest rattle can they could buy.
There is some protection using some browsing extensions and anti virus software. but there are still ways people can make malware undetectable, yes a strange .exe file or website attachment can be a virus or not, but I've seen blank extensions as well as other file extensions that are also malware. Also ransomware that goes undetectable and changes your windows permissions. It has gotten somewhat harder to vandalize these days but It is still vary possible and more common than most think. People even gone as far as calling themselves Microsoft and sending a malicious update to someone computer
what their deal is I do not know, some find it funny but how? Because your probably never going to see that persons face, and some malware designers have even broke their own laptops with their own program
I've known this to happen, I laugh long and loud too. Better them than me.
That said I have had almost zero trouble with malware and the like just by being cautious. I understand how it works, how it spreads, and how to avoid it for the most part. The only time I have had any trouble with malware in the last several years was because it had piggy backed onto a file I had downloaded. With some decent antivirus and with some tedious registry edits later, problem solved. As far as ransom-ware goes it doesn't worry me. I have all my important stuff on an encrypted external HDD. The HDD in my PC is of no consequence. I can trash it and go with a fresh install.
As far as tech scams go, they're pretty obvious. As long as you know the cardinal rule. You contact them, they won't contact you first. I am by no means a trusting fellow, online or in real life.
As far as the "why" is concerned. that is an answer psychology may be able to provide.