The correct answer is very simple - because it works. They trust the Force, and it guides them to correct decisions, in multitude of situations.
Digging deeper: as to WHY that's the case if the Force is mutable:
Yes, the Force isn't immutable as an absolute thing, and Sith can change it in some rare cases (E.g. Plagueis or Sidious).
But that's NOT a common occurrence, and doesn't happen for random things.
As such, an individual Jedi CAN rely on the Force for a majority of decisions.
As a good out-of-Universe analogy, consider this: Newton's laws of motion are wrong according to Einstein's both Special and General Relativity.
BUT, for a majority of mechanical calculations that most humans do and rely on, relativity effects are so insignificant, that they CAN rely on Newton's laws, without worrying that they are actually NOT 100% correct if you account for relativistic effects.
Also, when it matters, they DO know when they cannot rely on the Force, such as Yoda indicating that they can't predict the future during the events of the prequels, precisely because the Sith clouded the Force.