In mathematics, it is indeed possible to be accepted to a PhD program without a bachelor's degree, but only in special cases.
First, the person (the candidate) has to be exceptionally precocious and gifted with mathematical aptitude.
Second, the person has to apply to a very strong PhD program - the kind where the math faculty might have enough sway to convince the university to accept the person. At non-elite schools, the graduate college is likely to veto anything like this. And extremely strong letters of recommendation will be needed.
Third, the person must have at least one strong faculty advocate at the destination university who is able to sway opinion to get the person accepted.
As you can guess, this is not something that happens very often.
And that is for the best. It is a serious risk for a school to accept someone to a PhD program who does not have a bachelor's degree - perhaps the person will fizzle out. Worse, perhaps the person would have been able to complete a PhD if they earned a bachelor's degree first, but they ended up not earning the PhD when they were accepted early to a PhD program. For these reasons, it takes a truly exceptional candidate - more than just "seems able to get a PhD" - to convince a school to accept them to a PhD program without a bachelors.