i was thinking wouldent they be able to rebuild because mr.house has the strip look like building you see today but most things outside of it are destroyed so why havent they rebuilt houses and easy stuff to make?

Maybe due to lack of cooperation. You can't just have people simply build something. You would need the citizens of New Vegas ( which some live in the sewers) to build. The Securitrons are already occuppied and so that means Mr. House would need to force everybody to clean up the place. Which the destitute population don't seem able or willing to do.

House was only able to rebuild with an entire army of robot assistants, and Las Vegas wasn't nuked so the job was less difficult. There aren't many bulldozers and cranes to go around amongst the general population of the wasteland.