How to show attribute dropdown in add to cart form in Commerce 2.x
You have somehow disabled the proper widget. Go to the "Order item types" page and click "Manage form display" for your order item type, the widget for the "Purchased entity" field should be "Product variation attributes".

I would love to find out how your system ended up in this state. Was this a fresh install? Did you create a custom order item type? Let us know in the Commerce issue queue.