It's easy to become unclean, and it takes a lot of work, a lot of ceremony, and sometimes several days to become clean.
I don't understand the ritual or the rationale in Numbers 19.
Is this to show us that we can never fully attain purity?
That God is so holy, so separate, that in some ways we are always unclean?
Think of Isaiah 6, when the prophet "saw the Lord seated on a throne, high and exalted, and the train of his robe filled the temple" (6:1-7)
His response? "Woe is me. I am ruined! For I am a man of unclean lips . . ."
God is so holy, so awesome, that even though he was ceremonially clean, Isaiah knew he didn't measure up to God's standards.
Because of Christ, we are forgiven.
Because of Christ, we can approach the throne of grace.
Because of Christ, are clean.
Only through Christ can we be transformed to God's standards.