In other superhero shows. A superhero character keeps the identity a secret to protect themselves and the people that matters to them in their personal life. So their enemies won't target them.
However nearly all of Ben's enemies knows about the Omnitrix and his alien transformations. Even during the Original Series and Alien Force. In most of Alien Force Ben doesn't even bother to hide to turn into a alien.
Why did it took a obsessive fan like Jimmy Jones to reveal Ben's secret by the time of the events of the first Ben 10 Ultimate Alien episode. Jimmy exposed Ben's secret thinking he's doing Ben a favor.
Why didn't any of Ben's villains just revealed Ben's secret publicly to create stress and new obstacles in Ben's life and making it harder Ben 10 to help people.