Because we have many rich people and corporations willing to spend a lot of money to make sure they don't lose their cases. According to Wikipedia, Aleynikov was making over $1m per year at a Goldman competitor after he left.
And it looks like he got his money's worth. There was no dispute that he took the code. His "open source" defense apparently fell through, and a jury convicted him. He only won on appeal because he convinced the Second Circuit that software wasn't a trade secret within the Electronic Espionage Act, something which Congress went and explicitly my added in response to the court case.
So then your parent's question degenerates to, why does spending more money correlate with winning more cases? Which is a much more interesting question.
1. Money is a means of resource allocation. More money, more resources.
2. War of attrition. Raising costs on the other guy makes settling or coming to some sort of settlement more likely.
3. Scorched earth. GS may be trying to send a message not so much to Alyenkov as to any other programmers who'd be willing to entertain a similar stunt. See recent coverage of Amazon's frequent, but nearly entirely unsuccessful, pursuit of noncompetes against former employees.
the fact that we had prevailed against Amazon also gave these engineers the confidence that we could and would do so again -- and ultimately, it didn't prevent anyone from matriculating. It did, however, have one lasting effect: the engineer that was pursued went from thinking fondly of his years at AWS to hating AWS and Amazon with a white-hot passion that still burns today. In the end, enforcing a non-compete is like erecting a Berlin Wall: if you feel you need it, you have much deeper problems...
And it looks like he got his money's worth. There was no dispute that he took the code. His "open source" defense apparently fell through, and a jury convicted him. He only won on appeal because he convinced the Second Circuit that software wasn't a trade secret within the Electronic Espionage Act, something which Congress went and explicitly my added in response to the court case.