There are multiple aspects involved here.
First of all, the term אשת חיל is wrongly translated as "woman of valour". It really means a female warrior. Indeed, there have been numerous female soldiers in the past (Devorah, as you mentioned, was one of them), and the Israeli army, for one, has numerous female soldiers - some are sergeants. So, there's no problem finding female warriors.
The problem is finding a female warrior that has all the characteristics that are described in the rest of that chapter. For example, there;s a verse that says:
ידיה שלך בכישור וכפיה תמכו פלך
(My loose translation:)
She stretched her hands to the mill and her palms support the spindle
This implies skill at sewing or knitting or weaving; perhaps grinding. The point is, that a warrier doesn't have time to be at home doing this type of domestic work if she is on the battlefield. This is why King Solomon wrote an entire chapter and started it with the question, "Who can find such a woman?"
As to WHY King Solomon wrote about something that doesn't exist - well, perhaps, that's one of his many wisdoms that makes him wiser than the rest of us. This requires far more research than you can fathom.