The key here isn't a screening process for themes and plugins available through a WordPress website. The themes which have the back doors are typically pirated

The key here isn't a screening process for themes and plugins available through a WordPress website. The themes which have the back doors are typically pirated versions of paid-for themes which are never going to get into a WordPress app store. People are always going to pirate themes, people are always going to install themes from dodgy sources when they are free.