Why a lot of engineers don't care about a good and complete soil investigation?

In my opinion, if given enough time and resources, engineers would naturally want every factor affecting the project be considered, measured, and studied. However, due to some budget and time constraints of most projects, engineers are, for some instances, forced to just make estimations and assumptions instead of thorough investigation.

Being very familiar with the soil in the area.

Some Engineers rely on their experience and sometimes worst case scenario so no matter what the results of an extensive soil study they will still use their standards base on their experience and to make sure there are no surprises. I believe if its first time working on the area and no data about the land formation and soil composition then yes, specially where structures will be placed.