you're wondering why, and you are right to do so: this explanation (though leading to the "desired" result nevertheless) is complete rubbish.
of course the solid plays a role in equilibria involving named solid: else there won't be no equilibrium, would it?
and yes, the "surface" of the solid plays an important role for everything happening, and of course this is part for the respective K-value.
Fact of the matter however is, that the "concentration" of "surface" effectively next to doesn't change in a relevant manner *) , and hence is already "incorporated" in the K-value as a fixed factor.
this is an abbreviated way to explain at it. If you look at it more detailled, you'll see that it's in fact a number of interlocking trends, however mostly cancelling each other out in result. hence, the dependency of equilibria from the surface of named solid is quite small, so we neglect it.
... just like we do it with the real (!) concentration-change of water in reactions involving production/consumption of water (think of pH calculations, for example)