In other words, you're estimating something concrete — a fixed set of features. To insist on meeting an estimate at a fixed deadline is to commit to building that set of features. That set is bound to be wrong, however, so the estimate is bound to be meaningless. Committing to the deadline is effectively a commitment to build the wrong product.

I'm not sure about you, but in my "real world," building a product that nobody's going to buy isn't a particularly important goal.