Partly it’s because far fewer people are watching broadcast TV networks such as NBC than in previous decades. The highest rated shows nowadays have far fewer viewers, because there are so many choices with cable TV channels, internet streaming, You Tube channels, etc. to view.
So the top rated shows nowadays have far fewer people watching as a percentage of the population.
Cursory review online shows that the top primetime show this season is NBC Sunday Night Football, with 18 million viewers on average. That’s only about 5% of the population of the US watching.
I think late night shows are dated. The last great ones were Leno and Letterman. Nowadays people would rather watch Mr. Beast on YouTube instead.