We can endogenously produce it by converting it from ALA, but that conversion is very inefficient (5 percent), it doesn't result in the most bioavailable form

We can endogenously produce it by converting it from ALA, but that conversion is very inefficient (5 percent), it doesn't result in the most bioavailable form (lyso-DHA) and most sources of ALA have high omega-6s. Not to mention there's going to be significant individual variability regarding the capability of converting ALA. Likely, some are going to do a great job of it, and others not so much. Just look at how much heterogeneity there is in the endogenous production of gut myrosinaise for example. Our bodies were designed to consume some fish, even if we can do without.