You may know more than them, yet you merely stand on the shoulders of ever taller giants. Discounting the expense and difficulty of knowledge at the time isn't very illuminating.
I'm not disputing that. I'm responsible for zero new discoveries and if some of these polymaths were alive today, they would be far smarter than I am. However the "a lot" they knew about dozens of subjects combined is less than what somebody who is learned in one of those subjects would have to know today. It's impossible for somebody to be working on, say, string theory and also be an expert in two other subjects. There's simply too much to know.
I mean, look at Elon Musk and John Carmack. Pretty good counter examples to the narrative that you can't be good at more than one domain. Yes, they're very smart, but they're also very focused and driven individuals.