Why a ‘New Earth’ If We’re All Supposed to Be in Heaven?

You’re referring to what is presented in Revelation 21:1-5. In short, the phrase “a new heaven and a new earth” refers to what heaven will be like for us as resurrected, glorified human persons. That is, a three-dimensional reality that thus accommodates aspects of our physical earthly experience, though in a glorified manner.

For more on this subject, please watch this short video presentation by Jimmy Akin.

For further insight on the Church’s teaching, please see CCC 1042–1050.