I can't understand it either, I'm absolutely exhausted from _one_ day of biking. But check out things like https://en.wikipedia.org/wiki/Trans_Am_Bike_Race and the https://en.wikipedia.org/wiki/Transcontinental_Race for the really extreme stuff.

There's a great documentary about Transcontinental race from 2015 at https://vimeo.com/ondemand/transcontinental which is well worth watching. Seems to be behind a (cheap) paywall now but it's well worth it as an intro to how it's done.

It's absolutely mesmerizing to watch for regular crazy people (as opposed to the crazy crazy people who partake) and there is some great behind the scenes footage on youtube. These races are great to watch in real time because you can follow every GPS tracker and since each rider chooses their own directions it can get really exciting.