There is no canon explanation for this, actually it directly states how you're getting
crème de la crème of what humanity has to offer. No, seriously.
I actually addressed this in the
X-Com Files:
Our field operatives are normally recruited from various government agencies that focus on investigation or intelligence. Most natural candidates are those who were already exposed to paranormal activity in their previous employment. As professionals, they are trained to work in top-secret environments and were deeply screened for loyalty. They are experienced investigators who have basic training in firearms and a good understanding of field equipment. Many of them practice martial arts to some degree and they generally are in good physical condition. However, they are not professional soldiers, so any advanced military training must be provided by X-Com when necessary.
In short: the basis for recruitment is not combat abilities but exposure to the alien stuff in their previous line of work. It's not canon of course, just a handwave I came up with.
