The legal system makes it a priority because the thinking is it allows children as much of a biological family as is possible in their circumstances. Sometimes it just isn't feasible to keep all siblings together, but then a solution is sought where the adoptive families are willing to make the ongoing sibling contact and bond a priority. I've seen cases where children are split between two aunts, an aunt and an uncle, some cousins to ease the split but keep it family. I've also seen where adoptive families unrelated to the children but related to the other adopted families take the children and maintain the family bond in that way. Generally in cases I've seen where siblings aren't placed for adoption in a single household it is because of a large number of siblings, or multiple siblings with very special needs, or a large number of siblings who may share a common parent but for convoluted circumstances have never lived together as siblings. Plenty of times kids removed in abuse and neglect cases have lived together like siblings for a period of time and in fact are not biological, adopted or step siblngs -- yet have bonds as if they are. Because they do not share common biological parents it can be very difficult trying to keep their legal cases on parallel courses in the event TPR occurs and a possibility exists of placing them for adoption together.
I get that in black and white on paper it all seems so common sense, but sometimes things just don't all line up the way we want them to in these cases. I think it would be truly shocking to a lot of people, even a lot of people who've been on the waiting end, hoping to help a child(ren) they love have a stable and loving home just how hard it is for the case workers, the attorneys, the referees, the judges and everybody else involved in this type of work to deal with trying to make these situations work out well. I have dealt with some case workers several times over the years who I thought were honestly useless and didn't care about the kids and weren't doing what needed to be done in cases I was involved with. More than once circumstances revealed to me just how much some of these same workers were invested in these cases, and sometimes how badly they were affected when things went sideways.
When I got involved with these kinds of cases about a dozen years ago I found some of the things I thought I knew upended by the realities I discovered. Even in cases where I know kids desperately need their parents' rights terminated so the children can have some stability and be allowed to flourish, it is still a very strange and sobering thing to sit there and listen to a judge or referee make a ruling that a parent's rights are terminated, or sit and listen while a parent goes through the technicalities of the law to relinquish their rights voluntarily. I've sat there happy and relieved for the kids, completely disgusted and appalled by the parents, yet my heart felt like some queer, semi hard cement because I understand those ties are severed and somehow it always feels against nature to separate children and parents. Only one time did I only feel joy and relief for the child with no heavy heart about the bond severed, but that's a story too painful to share.