How would that excise the viral sequence from the already rewritten dna in cells previously infected, rather than just slow further new uninflected cells from being infected?
The point is, in the presence of a retrovirus with its reverse transcriptase trickery, an organism is a chimera, populated by dna-distinct cells (for example, never infected cells vs cells previously infected and thereby holding modified dna, who would send out new virus during their own replication).