More specifically, it was arrived at as a target for labour rights campaigners after the 10 hour day had started becoming common. Owen formulated the famous "eight hours labour, eight hours recreation, eight hours rest" slogan in 1817, but it took half a century for the 8 hours day even to become the main goal for labour organizations most places (because they were still fighting for the 10 hour day, or even the 12 hour day), and it took more than a century from Owen's slogan for the 8 hour day to become widespread.
Since then labour organizations in many countries have set goals for shorter work days, and many countries have seen the work days shortened further to various extent, but given how long the fight for the 8 hour day took, we can expect it to take a long time.
But as you say, it has absolutely nothing to do with 'optimal'.
Since then labour organizations in many countries have set goals for shorter work days, and many countries have seen the work days shortened further to various extent, but given how long the fight for the 8 hour day took, we can expect it to take a long time.
But as you say, it has absolutely nothing to do with 'optimal'.