Think about it
We have a tag like TAX! How many different taxes there are and the definition changes very often! We have tags like DATE or TIME! Give me a break. How we did handle that in past, yes, metalanguage was allowed in many systems even in 70's but any definition had to be documented and accepted in context. Not a big deal, takes two minutes in a well defined infrastructure and then you can use it, references built, correct display and store formats and calculations bound, etc. But giving a free hand is no, no in business. Why create problems when it is easy to avoid them?