And why could the UK (and the US in 1917) not?

In this extra video, The Bloke has a bash at explaining the fundamental difference between the US and UK, on the one hand, and France and Germany on the other, when it came to organising their militaries in the pre-WW1 period.

Basically, the French and the Germans could put a gazillion men into the field in 1914, whereas the UK couldn't, and the US couldn't do it either in 1917. Why is that? What did they do differently?

The difference is illustrated with the case of Lazare, a French Zouave conscript, in the 4th Regiment of Zouaves.

Might be interesting with the increased interest in world war 1 with the release of Battlefield 1...