Hence, counting back from 537 BCE (the year the Bible says the Jews returned home) for a full seventy years, we arrive at the year 607 BCE
The real question is:
Why do Jehovah's Witnesses choose to accept 537BC and count backwards, instead of accepting 587BC and counting forward?
The evidence for 587BC is much stronger than the very credible evidence for 537BC. If you have to pick only one, it would make sense to pick 586BC and count forward.
Unless of course you have based your entire theology and claims of authority on 607BC and 537BC....