Wayne, WikiTree requires at least one of those dates because of search needs. These include enabling users to scan through a long list of profiles with similar names in order to find the one they are looking for. It also enables WikiTree to display a list of potential matches filtered by similar dates when someone is adding a new profile. There are a few other uses, as well.
This does not mean that you *MUST* know an exact date for either birth or death - you can use just a year if you know that or you can even enter an estimated year. If you know the birth year of the person's parent, you can guess something like that year plus 18 and mark it "after this date". If you know the death year of a parent, you can guess that year and mark it "before this date". If you know the birth year of a child, you can also make a guess at the parent's birth or death date the same way.
If you can't make even a reasonable guess at one of these dates then WikiTree does not want the profile entered until you do enough research to be able to either find a birth or death date or make an educated guess at a before, after, or about date.