It's not, the probability of a SpaceX rocket spectacularly exploding is somewhere around ~1%. They'd need 860 more problem-free launches to hit ~0.1%.

The probability of a truck spectacularly exploding on its way to the launch facility is way less than 0.1%. In fact, it's astronomically lower than 0.1%.

> They'd need 860 more problem-free launches to hit ~0.1%

For what it's worth, SpaceX claims that they intend to reach this level of safety by literally performing thousands of launches. Time will tell if they can do that.

You can't calculate probabilites that way when the process itself is improving.