People can work as much or as little as they want, once they make tenure. It's not that different from the startup model: hard work for a few years up front, then cash in your chips, with 2-3 decades of guaranteed income until government retirement age.

Having worked in academic research and co-founded a startup I would admit that the working hours weren't that different. However, in a startup you have a much wider set of pressures - making payroll at the end of the month, keeping customers happy, investing your own money to keep the business going, hiring/firing/managing people etc. (it's a very long list).