Because incomplete data is a fact of life.

Companies that only do business when all data their database has a column for is known, will soon not do business anymore. Many fields are mandatory, but not all are.

If a company collects data about household situation and income of their customers, for statistical analysis and maybe some data mining and targeted advertising, and I leave the fields for number of children and monthly household income blank, would you really enter both as 0? I do have children. And I do have income. I just refuse to share that information with companies that have no need for this information, and no legal basis for asking me about it.