That may be question of national security i.e. ability to defend against adversary. Let's imagine 30 years from now. World is not yet united and peaceful place. Nation focusing on social nenefits, leisure time, innovation and freedom could be as economically and technically advanced as other nation focusing solely on full employment.

But they will be substantially different when facing difficult realities of war: those benefits as well may be cut and those people should go to factories, computers, fields, whatever occupation current war needs.

How do you explain a whole generation of people that their allowance will not be paid? How do you tell them that their occupation will be unpleasant? How do you train millions of people discipline, relevant skills, command-and-control?

No major nation wants to lose next war, so we are locked in full employment race.

(sorry for bad english)

So you're saying that people should do busywork now, just in case we need to go to the factories to support tomorrow's war? Forgetting the fact that the factories are already automated, and going to war won't un-automate them, war is ever less likely and smaller in scale as global prosperity increases.

Which world do you live in? It seems like we are slowly inching closer and closer to world war 3.

Americans are more and more averse to going to war, which is why the USA is sending drones instead of people now. The situation is similar in Europe. Not so much in the other parts of the world, though.

A reasonable explanation could be proximity: When the war takes place in their own country, people are more likely to pick up guns instead of waiting for drones to be sent.

I'm guessing you're young? Seems like we were a lot closer to WW3 in the 1980s, and it's only been getting less likely since then. The US hasn't had a draft since 1973.