Because we live in this world and wish to be happy and effective and prosperous in it, and for this purpose, teaching scientific truth as best we understand it is MUCH better than teaching somebody's superstition.
A few of things:
First, happiness, effectiveness and prosperity only exist under the evolutionary illusion. They can be nothing more than natural selections. To assign meaning to them is erroneous.
Second, when you say MUCH better, what is the basis of that value judgement? Much better, how? Evolution is undirected - their is no ultimate goal or purpose. In fact, there is no such thing as "purpose". To suggest otherwise is to purposely forget what has been learned. We could teach students anything and, ultimately, it makes no difference.
If all humans died tomorrow, so what?