Why the Tillman maximum battleships were drawn

Why did American designers draw Tillman battleships with such extreme armament?

To answer congressional demands for an ultimate battleship and make its cost apparent.

The enormous Tillman proposals are explained as a response to congressional pressure for an ultimate battleship. Producing such a design could expose the cost of the demand and allow naval designers to return to practical work. A quoted commenter contrasts the Navy's preference for incremental improvement with a senator's demand to stop requesting annual upgrades. The comment also attributes Tillman's opposition to fears that colonial expansion would harm his state's agricultural interests. That account of his motives belongs to the commenter and is not independently demonstrated in this exchange.