Why isn't Prince Philip next in line for the throne?

They are in line, theyre just in a position where they can be pushed back due to primogeniture. Currently, Prince Andrew is 4th in line, Edward is 7th.

The Queen's eldest son is first in line to the throne, Prince Charles. In 1980, Prince Andrew would have been second, as her next eldest son. However Prince Charles himself had a son in 1982, and another in 1984 (William and Harry) and these go before their father's siblings in the line of succession.

In the same way, Andrew's own daughters, Beatrice and Eugenie, pushed his younger brother Edward back, and in turn, if Prince William and Catherine Middleton have a child, this child will be next in line after William, before his brother Harry.