Why is Add to Cart faded on my variable products until shoppers pick an option?

That's how Variable products work. Wcart can only add an exact variant to the cart, so the shopper has to pick a value for every option first.

Until they do:

• A hint under the options says, for example, Select Size & Color to continue.
• Add to Cart is shown faded.
• Clicking it shows Please select all options.

Once every option is picked, the button becomes normal and the price changes to that variant. If the chosen variant has no stock, the button changes to Out of Stock instead.

There's no setting to pre-select an option for everyone. To save shoppers a step from an ad or banner, link to the product with ?variant= added, for example ?variant=Size:M, so that option is already chosen.