Light does bend in a gravitational field. why is this bending not taken into consideration by surveyors who use laser beams as straight lines?

Because the effect is not big enough to be noticeable.

Explanation:

The light is bent by gravitational fields, but the bend is not that big unless we are talking about objects with a massive amount of mass. To be noticed the bent, you need to stay far away from the object that causes the blend and the object also needs to be far away from the source of the light. For example, you can observe the blend in the light of a far-away star when the light travels close to the sun to reach earth, and the deflection will be around 1.75 arc-seconds. The deflection occurs also with light beams on the earth but the effect is too small to be taken into consideration.