And there are lots of people born before 1800 whose parents will never be positively identified. Unless a family owned property, left a will, or involved guardianship there just aren't many records. In many cases the best you can do is outline circumstantial evidence - "John Doe married Mary Roe at place xxx in 1818. In 1820 John, age about 25 is on the census at place xxx, living next to James Doe, age about 45. James is on the census at place xxx in 1810 with a son aged about 15" - that might support a relationship. It's just way too easy to assume that a same-name person is a relative.