On the other hand, you don't usually get hit by random pieces of debris traveling at mach 25 [1] on Earth.

And on the other-other hand, the things that go wrong in spaceflight usually go wrong on launch or reentry - the parts where you combine the extreme conditions of space with the unpredictable extra nonsense of Earth. Like, say, a cold morning making an O-ring brittle or aerodynamic stresses ripping you apart on reentry. Only three deaths - the crew of Soyuz 11 [2] - have actually occurred in space proper; all other deaths have occurred within the Earth's atmosphere. (Although Apollo 13 came close to adding to that list.)

[1] LEO orbital velocity is ~8 km/s, mach 1 is ~300 m/s, although meteoroids necessarily strike at much higher velocities than that because they're accelerated to at least Earth's escape velocity on descent.

[2] https://en.wikipedia.org/wiki/Soyuz_11

I suspect that it isn't the unpredictability, rather it is the acceptable margin of error.

Say you have a 1500 kg car with a 100 hp engine, which starts from a standstill and runs for 5 seconds. How fast will the car be moving at the end of this run? This depends on how much friction the vehicle experiences, and is difficult to accurately predict. Try a similar calculation with a rocket in space, and you can calculate its final speed with a very high degree of certainty.

But now consider the potential impact of a malfunction: if the car engine fails to start, you might be inconvenienced, but you will be fine. On the other hand, if the rocket engine fails to ignite in space, you can't perform necessary orbital maneuvers and you might be in deep trouble.

It's something about environment on Earth being fundamentally hospitable to human life, and environment in space being similarly inhospitable. Because of this, malfunctions and unpredictable system dynamics on Earth are most often just minor inconveniences, while malfunctions in space / underwater / deep underground are disastrous events.

Think about it this way: what happens if your car won’t start in the grocery store parking lot? What happens if your lunar ascent module’s main engine doesn’t ignite on the moon’s surface?

One is mildly inconvenient, the other is deathly inconvenient. Yes, many aspects of the extraterrestrial environment are predictable, but the consequences for being wrong are far greater.

And as another has noted, most deaths during space flight actually occurred in the atmosphere anyway.

The toilet on the shuttle dumps urine overboard but keeps feces in a tank. They just float around in there. There is a mechanism that gets actuated once or twice during flight which spins a net attached to an arm through the tank, to catch all the feces, and then smashes them up against a wall of the tank.

The mechanism to run the net can break. It has broken during flight before. At that point you have to put on an arm-length glove, stick your hand in the tank, then smash all the feces up against the wall of the tank manually.

You need to do this because at a certain point if you open the toilet to use it there's a very good chance you'll get something floating _out_ of the toilet.

Free floating zero-g human waste is an absolute nightmare.