As the question suggests. 
Probably because no one has had any need for it. It’s a uncommon operation and not even javax.vecmath has a shear functions as far as I can tell.
I thought shearing was done a lot when you wanted to show stress/strain or even flags being waved by the wind.
Nah, you’d do that just by dividing your flag up into a grid of quads and then perturbing the vertices in a clever way. Or use a shader.
Cas
You can get a shear effect by using non-orthogonal basis vectors for the matrix, as long as you don’t get too carried away.
Oh yeh. I didn’t think of that.
In the end though it doesn’t matter as Princec said that anything requiring a shear is best done on the GPU due to performance reasons.
And where exactly do you think the heavy lifting of a shear operation is actually done? Sheers can be useful for cheapass shadows though.