QUESTION: Why aren't fields independent of entities?

Hi,

I am not sure if this is the proper place to post this question, so please direct me to the correct group if this is not.

I am curious why fields in Drupal cannot be created through the UI without first creating some entity (i.e. a node)? I have seen the ability to do this in other content management systems, which make it easier to setup a content model where fields are being shared across content types. Not saying that is the best way to architect a CMS, but it seems to be a common feature of other systems that also allow for structured content models.

Thanks for your responses!