Why is it that every RDBMS insists that you tell it what the max length of a text field is going to be... why can't it just infer this information form the data that's put into the database?

I've mostly worked with MS SQL Server, but every other database I know also demands that you set these arbitrary limits on your data schema. The reality is that this is not particulay helpful or friendly to work with becuase the business requirements change all the time and almost every day some end-user is trying to put a lot of text into that column.

Does any one with some inner working knowledge of a RDBMS know why we just don't infer the limits from the data that's put into the storage? I'm not talking about guessing the type information, but guessing the limits of a particular text column.

I mean, there's a reason why I don't use nvarchar(max) on every text column in the database.