YOU could have googled the reason why and but then we couldn't call you out on just how STUPID you truly are.
The Constitution says EVERYBODY gets counted, because it's necessary to apportion resources according to POPULATION. If non-citizen slaves hadn't been counted, then the South would never had had "proportional representation", or received federal funding for their infrastructure.
Schools, hospitals, water treatment facilities, sewage treatment facilities, and other service all humans require, citizen or not, are all allocated based on population.
If MAGA wasn't so completely dependent on the lies Trump tells them, they would KNOW that.