Why do the names ethene and propene not include the position of the double bond?

Answer 1

There is only one possible position for the double bond.

Ethene ....... H2C=CH2

Propene ..... H3C−CH=CH2

If you rotate H2C=CH-CH3 then you get H3C−CH=CH2 .... It's the same thing whether you look at it from the front or the back.