When we see thou, thy, or thee in prayers, we need to remember that THOU was used for people you were closer to; it was less formal than YOU.
God & the saints are close to us, & we can communicate like friends: it's not we need ot be more formal with them.
It does. The plural 2nd person pronoun is YOU.
THOU is the 2nd person singular. YOU is also formal. Slowly, people got more formal, so used THOU less until some forgot it existed.