Many of those joining the forces are NOT poor, unqualified, nor badly brought up, but some people, male and female just feel more comfortable in an organised and structured environment. That is why many of those leaving the army go into other uniformed services like the police, prison service and security roles.
I know just one such man, now in his 40s. He left school with a good plate of O levels, chose not to A levels, but joined the army. He signed up for a short engagement, left with an exemplary record and joined the police. He has now been in the police force about 20 years and has reached a reasonably senior level. He is someone who is just more comfortable when he has clear rules to follow, in any aspect of life.