When we get called in for water problems the first thing we hear is the neighbor said that there is a stream running under the house.
We have never found a stream under any house in our years of doing consulting work in the area.
Its mostly due to bad grading or downspouts emptying near the foundation. Remedies may include redirecting downspouts or improving the grading. The standard remedy of putting drain tiles on the inside should only be used as a last resort