Because often you are not hiring a team, you only have so much work/budget. And if you do, odds are you will need to break up the team a short while afterwards for some other needs. And you want to merge them into existing teams anyways, to not lose institutional knowledge.

I think almost all "hire a team" hirings happen as aquihires - if the team has proven that they can deliver something over time - great, buy their company.

I dont see how ability to organise as a team could be done in a short enough time that you could evaluate them on anything other than "is someone in the group good at organising a team". At which point I'd probably just want to hire that guy anyways, because I know I can split them off and have him organise another team too.