Many because they stole company secrets, or actually hacked the company's servers and walked out with company documents.
Corporate lawsuits against individuals generally don't need grounds. The pain is the point.
What's groundless is this mentality. Please read Rule 11 of the Federal Rules of Civil Procedure (a rule that is also well-adopted in state courts as well). As fun as it is to hate on big tech, one should still be mindful of the fact that their attorneys aren't stupid enough to file patently frivolous lawsuits that put their own law licenses on the line.
Has any corporate attorney ever been disbarred for filing frivolous lawsuits against individuals on behalf of their company? It's certainly not a common occurrence.