This means that the rules for applying the tax to the cart have not been fulfilled yet.
In the eCart object, how is the tax rule defined? Usually, it is based on the billing state. The user does not enter the billing state until they click the checkout button and fill in the checkout form.
In most cases, you will not see a tax rate on the cart page, the tax rate will be calculated on the confirm page, after the user has filled in the checkout information.