Porcelain veneers are not considered permanent because at some point it is likely they may need to be replaced. If your bite is optimized before they are placed, veneers last for years as long as they are taken care of properly, and some patients never need veneer replacement at all.
Porcelain veneers are irreversible once the enamel is reduced to make room for the veneer. The enamel does not grow back so patients will always need veneers or crowns on those teeth.