Because it's not enough to just simulate things.

https://spaceflightnow.com/2020/02/28/boeing-says-thorough-t...

> Both errors could have been caught before launch if Boeing had performed more thorough software testing on the ground, according to John Mulholland, vice president and manager of Boeing’s CST-100 Starliner program.

> Mulholland said Boeing engineers performed testing of Starliner’s software in chunks, with each test focused on a specific segment of the mission. Boeing did not perform an end-to-end test of the entire software suite, and in some cases used stand-ins, or emulators, for flight computers.

Cheaping out cost billions and nearly two astronauts.