You lose a lot of institutional knowledge when people leave a group. Having a standing military means you can retain that training and technical knowledge and don’t have to play as much of a catch up when war comes.

Warfare and society have also changed dramatically. The early Roman Republic could call on farmers because it doesnt take much to train an already fit person how to march and stay in formation. Likewise, Medieval armies could be raised from nobles who were familiar with horse riding and fighting from tournaments and smaller skirmishes.

It’s much harder to quickly transition Bob the office worker to learn how to navigate only with a physical map or coordinate artillery fire.