Because the Dining Plan gets you a “combo” and Universal refuses to give you a soft drink with their combos, forcing you to purchase an all-day refillable mug along with your all day dining plan.
Therefore, every “combo” includes a shake. And they put it as the main offering on the menu boards so those with the dining plan feel like they’re getting a good deal.
Of course, you can order any meal not as a combo, but any chance to upsell you is a chance they’re gonna take, so non combo pricing is being left off the menus lately.