For the simple reason that he is missing the whole point of democracy which is that you can vote to change the system and people with his views are represented by The Green Party, just like the far right is represented by UKIP, as unpalatable as they are.
How do you know the Greens represent him? He might, frinstance, have strong views about the importance of nuclear power, or similiar. Just being leftwing doesn’t automatically make you a Green.
His point is, basically, if none of the parties speak for you, don’t valiadte them. Something I’m quite sympathetic to, having voted tactically for the least shit option in the past and then seen her take that as a massive vote of personal confidence rather than the reality of “we like the other **** less”
I’m not sure how effective it is- we seem to have accepted that low turnouts are just how it is, rather than a terrible proof of failure. But it’s not a stupid idea in principle, it might just be that things are broken enough that it doesn’t work. After all, we have a prime minister now with 36.1% of votes from a 65.1% turnout and essentially no representation in 3 out of the 4 parts of the UK, who’s never once acted like he has less than 100% support.