This paper explains it perfectly:
"First, the distribution of provirus-containing loci among taxa dates the insertion. Given the size of vertebrate genomes (>1 × 10^9 bp) and the random nature of retroviral integration (22, 23), multiple integrations (and subsequent fixation) of ERV loci at precisely the same location are highly unlikely (24). Therefore, an ERV locus shared by two or more species is descended from a single integration event and is proof that the species share a common ancestor into whose germ line the original integration took place (14)."
Constructing primate phylogenies from ancient retrovirus sequences
We find over 200,000 ERV's at the same base in both the human and chimp genomes. There are only a relative handful of ERV's in either genome that are not found at the same spot in the other genome.
It begs the question as to whether or not you care about the truth.