They more than likely made that decision to be able to style the label based on input state using the + sibling selector. I use that trick for literally every visible input now.
I think it's for material design inspired focus animations. :has selector wasn't a thing, so you have to use + sibling selector to target the label of a focused input
input:focus + label {}
label:has(input:focus) {}
[1] https://getbootstrap.com/docs/5.3/forms/checks-radios/