I don't think most people start out intending to steal $100M. A lot of times it's just some middle-class regular joe that hits some financial trouble or wants to buy something they can't afford and they decide to try it once, swearing it'll only be this time, and they get away with it. Later they start thinking about how easy it was, and they do it a second time. And then a third, and so on. Eventually it just becomes part and parcel of their daily routine. Clock in, pay some fake invoices, move some money around bank accounts, and then start shopping for luxury goods. They get addicted to being able to buy whatever they want, and even more importantly, looking rich AF.
Texas Monthly did an in-depth story a few years on a guy's years-long embezzlement scheme at the Collin Street Bakery in Corsicana that really delves into the why and the how. Definitely worth a read.
https://www.texasmonthly.com/true-crime/just-desserts/
Also, back to my earlier point, the dude didn't get caught for so long because he was basically the entire accounting department and had no oversight. He got found out almost immediately after they brought in an assistant.