It's the same reason the Spartans, for example, tolerated and implicitly encouraged infidelity on the part of their women. Anything to make more warriors.
Arranged marriages would have mitigated many of the potential negative effects. The fathers' goal would be to get their daughters married off to suitable partners. I'm willing to guess that if the male-female ratio was reasonably balanced, fathers would angle for an unmarried man or a widower, to whom his daughter could be the first, the primary and perhaps the only wife. But if the last couple of wars had gone badly and three quarters of the tribe's or village's young men had been killed, a polygamous marriage might be the best honorable solution, the alternatives being concubinage, prostitution or beggary.
Plus I was reading about the Yanomami in the Amazon, who to a certain extent practice(d) female infanticide. Naturally that means that when they come of age to seek mates, they can't find enough females in their tribe, so they raid neighboring tribes for women.
Feuding, banditry, raiding, abducting women--- that's how they live and die.
It's obvious they were (maybe still are) stuck in a hell of their own making.