ERVs and universal common descent- it doesn't make any sense
Yet in order for that to happen it means that an virus had to infect a gamete, that gamete had to get used to make an offspring, and then that offspring would have to pass down that genetic marker such that it became fixed in the population.
All that while keeping its sequence identity, over thousands and thousands of generations with recombinations and mutations happening all around it.
So the question should be why did the sequence identity remain intact?
The obvious answer is because it does something and most likely isn't a leftover infection.
Another question would be "are ERVs actually decayed versions- ie selfish genes- of a once full genome?"
Think prions which are leftover proteins which go on to infect the organism that ingests them.
IOW these ERVs, in their normal state, are just another regulatory sequence in the genome.
When the organism gets consumed, for example, if the DNA does not get broken down all the way, these selfish genes can then "take over" the new host.