The standing army is a bit of a historical aberration. There were militias, armed nobles, and guards - but service was more a type of tax or tithe than a civic duty. You got land from the king, and in turn agreed to protect it and support him against his enemies. Mercenaries were pretty common - pay for an army only when you need it, and get forces with experience rather than armed serfs. It was the rise of industry and to some extent fire arms - that lead to the need for specialist forces and large national armies.
Soldiers for hire became quite common after The Event. With the devastation of the industrial base, supporting advanced systems or large armies became quite difficult. Concurrently, a drop in population meant that that most settlements couldn't afford to have people serving all the time and not helping elsewhere. Although some form of protection against reanimates was needed, most surviving enclaves did not need to protect a huge area or force their desires on other nations which also meant that large national armies were somewhat unnecessary as well.
Around the same time trends indicated the time was ripe for mercenaries, the potential supply also sky-rocketed. Many soldiers found themselves with some equipment, but cut off from command and control. Those who chose to be raiders or simply give up the military life generally left their heavy equipment behind due to difficulty of finding resupply. Soldiers who wished to hold on to it generally needed to either find a way to link back up with the government or to make it profitable. This also was a ploy for personal security - working for a town meant stability and allies - wandering alone with heavy weapons made many consider that individual a threat.
Of course, not all soldiers for higher started as professional military. Many people with relatively limited skill sets or criminal records found that it was easier to sign on with a militia than pick up other skills. Others saw it as a path towards exploration and adventure, while still having the safety of numerous armed companions and steady work.
A number of non-military mercenary services also appeared after the event. One of the best known of these groups was the "Ambulance Chasers" - medical raiders. There were also reports of free-lance fire companies who would help inspect and reconstruct ad-hoc settlements to be safer places, college professors trying to teach (much like hiring Greek Philosophers in Rome), and assorted technicians assisting the creation of new electrical and mechanical systems. Retrieval Experts and reanimate/alien extermination squads were another common theme - though the latter often operated under a rather heavy stigma.