patriotism. i can't answer for europe but i can answer for usa easily, it's patriotism. the younger generations and in particular millennials and gen z look at the disastrous vietnam war, iraq war, and afghanistan war and have avoided military service. the world has invented this image of the us soldier in their heads that no longer exists, post ww2 america has lost every major modern war.  the youth of the 1960s and 1970s were famously anti wars, we have the hippie and beatnik stereotype from this generations who were forced into the vietnam war. a lot of people don't want to admit it but war time nations change the face of their population for decades. post ww1 and post ww2 ussr/russia lost so many men in war that their replacement generations were different. you bet your ass ukraine's post russia war will have a different demographic, one that might be forced into military service at age 16 like israel and south korea. america's post 3 brutal war losing streak got a lot of men thinking about why women aren't on that forced draft list by law.
United States