Indeed. And while I get your point that it's often more than 40 hours, the reality is also that an 80 hour work week (PhD + job) doesn't work for the vast majority of people. There are people for which it works, but chances are -- statistically -- that you, the reader, are not one of those. So a PhD being a full-time job, even if that means "just" 40 hours, is indeed a reason why full-time job + PhD does not work in practice for most people.
The main problem in academia is that worldwide those who work a 40 hour week seem to be far less common than those who work, or claim loudly to work, more than 100 (particularly at the earlier career points). I'm lucky enough to have gone beyond the point where I sleep in the lab, but I definitely have done so.
The OP should be able to do a doctorate part time, and in Europe that is definitely doable. The funding and the practicalities are very much subject specific, but it does take a long time -- and is very much a labour of love.
That should mean it's a 40 hour a week commitment, not that it's the only thing you do for half a decade or more.