Interesting take, and not entirely wrong either imo. Though, I think the real reason is simply that such a commandment wasn't necessary, because it was already implied from the very beginning. God gave Adam and Eve the mandate to care for his creation, and in conjunction with the fact that "Love the Lord your God" is the very first commandment (which means to follow his commandments), respecting the people that God created in his image would have absolutely unquestionable in the mind of the ancient Israelites.

The really hard to accept part is how this respect for what God made included the destruction of what is not of him, which included people. It's a very alien concept to us today in our culture. The important part is that what you read in the Bible (esp. the Old Testament) cannot be taken at face value. Everything is seeped in historical context that often makes things seem at a glance to be the opposite of what they actually are. The translation from Hebrew and Greek compounds this problem.

TL;DR: If you want to take solace in confirmation bias, it's not hard to do, and to blame you for doing so would be incredibly hypocritical of me. Remaining truly objective is the most grueling exercise in self-awareness and accepting uncomfortable possibilities anyone could ever undertake.