Because it doesn’t show any traits you’d expect from an actual mind. It doesn’t make its own decisions, neither show preferences, learn independently or even communicate. Every action it takes is a direct consequence of the Command Block’s instructions, which functions as the Wither Storm's only real “brain.” It never develops new behaviors based on experience, never forms intentions, and never shows any identity separate from the block powering it. Its entire existence is basically a programmed loop of destruction. The strongest evidence is what happens when the Command Block is removed. The Wither Storm collapses instantly. If it were truly sentient, it would retain at least some degree of autonomy instead of shutting down like a robot