It's not. Often the SSN alone is used for identification, which is why it's such a big deal if it leaks somewhere. And random companies can use it as well afaik, increasing the odds that it leaks online.

https://sites.psu.edu/rclaes/2020/10/15/rcl4-advertising-gone-wrong/ Here's an interesting article about a CEO of a company specialising in identity protection being so confident in his services, that he printed his actual SSN in an advertisement.

He had his identity stolen 13 times shortly afterwards.