Because they want it. Literally the only opinion that matters is that of the people actually involved in it and they want nothing more than racing, in this case, the Dakar or the Isle of Man TT and similar road races. It's their life, that's what they're breathing, it's what gives their life meaning in their eyes and that's what they're living for. They all know the risks, every single one of them, and every single one of them accepts it.
On our Dutch sports website we had an interview with a Dutch racer in the Isle of Man TT in 2017: readers could ask him questions and he would answer them all - he was the boyfriend of one of our editors, hence the easy connection. Part one of his answers had been posted on the morning of one of the races. On the forum we were all following the live timing until at some point he didn't come through one of the checkpoints. News broke pretty soon he had had a crash, but reports were not clear. "He's ok, but his bike is destroyed" to "It's not looking good", all we knew for (pretty) sure was he crashed.
Not much later I got a PM from the editor saying she had heard he had passed away, and later that afternoon it was announced on the local radio. In some sort of weird coincidence, the first question he answered in the part that had been published on what turned out to be the day of his death was "First of: which colour coffin would you like to order?", with his answer being "Gold of course! #winning"
His answer to a question about whether or not he thought about the danger tells a lot. "Of course when you're lying in bed at night, you think about it a little. After all, it's a dangerous track and stuff happens. On the other hand, the big risk is exactly what makes racing right here so awesome and so special...if you can't deal with that, fine, but go do something else. I'm doing this because there's nothing better in the world than racing right here. I'm living my dream and for me, nothing in the world is worth more than that."
And about preparation, some advice for people who want to do this: "Do your homework! The very last thing you want is to be riding your first lap here, only to realize you didn't study hard enough."
All in all, that says enough I think: the riders want it. They are batshit crazy and they know and love it.