I feel like SSN is a big liability with little benefit over a different primary key like your phone number. SSN would be important if you're signing a mortgage, but these people are tracking how often you buy dogfood and pregnancy tests.
> I feel like SSN is a big liability with little benefit
Your feelings on the matter are quite irrelevant -- they want every bit of data about your financial life as possible. Including your mortgage, income, etc. The SSN it the ultimate primary key. You can't even get a new one if your identity is stolen. The more often they can link that to an email address, phone number, etc, the stronger the signal they get from sources that only have those bits.
Phone numbers change all the time. Some people have more than 2 phones. Phone numbers tied too names become less trustworthy when names change.
The reason an SSN exists is the same reason why it is one of the the best identifiers of a person in the US.
Nation state and domestic hackers look for these as a highly prized commodity for this reason as well. They can be used to find other records containing people. You may recall the Chinese hackers who exfiltrated the SSNs of US military personnel not too long ago.
Considering that there are only 1 billion possible SSNs, that seems like it would be easily reversible simply by pre-computing a lookup table of all 1 billion hashes.
