That’s a sharp question to ask. Glad you didn’t miss it.
So if you look carefully, this is a reenactment of the fall of mankind in Genesis. When God had given Adam & Eve everything in the garden for food, but they have to go for the one thing that is forbidden. To take for themselves what they see as good for themselves, not what God think is good for them. The verse 2 Sam 12:8 means that everything in Saul’s kingdom has been given to David, was it not enough that he has to take of the forbidden fruit and sinned just to do it?
However in case there is any doubt on God’s stance on polygamy, here is a decree for a king, given way before they were a kingdom and had a king.
Deut 17:17 NASB
He(the appointed king) shall not multiply wives for himself, or else his heart will turn away; nor shall he greatly increase silver and gold for himself.
Here, God is teaching that a king’s position is to serve the people, not “for himself”. So God made it loud and clear enough regarding polygamy, and even added a warning. The kings indulged themselves in polygamy, like Solomon, the greatest offender of all, and his heart did turn away eventually, but in no way it means God is pleased with it or endorses it.
In fact, all biblical accounts of polygamy only goes to show that it never ends well. The households will be full of strifes and infightings, like Jacob’s. David’s household is a bigger mess, with Absalom literally shaming his father by setting up a tent to lie with all his father’s concubine in the sight of all Israel (2 Sam 16:22), thanks to Bathsheba’s grandfather, Ahitophel’s advice (2 Sam 11:3, 2 Sam 23:34). Even Abraham taking a concubine just to have a child, created one of the largest thorn against Christianity til today. LOL
So reading all these accounts should serve as a warning, to discourage us, and show us the ramifications of a polygamous household. God’s design is for one man one wife household, as @kiichimae and @KMac’s articles clearly pointed out.
Hope that sheds some light.