and difficult to answer. I suspect you would want a simple answer, one that would satisfy your curiosity and explain everything. It's a bit like the philosophical question, "Why are we here?"
There is a simple answer. BECAUSE THEY CAN!
But of course, that is too simplistic and is largely no longer the case.
Perhaps I can describe my own experiences. When I started with internet capable computers in 1995 the internet was a much simpler, cruder, and more innocent experience. Besides email and a few text based web sites, there wasn't much else. No music, no videos, not even any images, no online commerce. Most of the information I sought, on computers, or other interests I had like Astronomy and space exploration, I found in Newsgroups. I had very few email contacts.
But already the 'script kiddies' had started, sending emails whose contents could be converted into executable files which, when you ran them, would display some message or other on the screen as a joke. That soon developed into executables that could do damage, like deleting files, or trashing whole systems. In my opinion that was the start of viruses and we had to learn very quickly not to trust anything that didn't look right. There were no firewalls, and no anti-viruses, in those days.
Why did they do this? Because they could. They were learning about the functions of computers and how to code. It was fun for them.
It's developed since then. It is big business, and I mean, HUGE. It is now largely criminal in nature to steal information and data, to steal money, or to take down computer systems.
Why has the criminal side got involved? Because the rewards are counted in the millions of dollars.
There you go. That's why.