Identity theft happens because of the data these companies sell (or even give away).
Sometimes you will be asked to provide past living addresses (or pick the name of a street your lived on before) to prove your identity with some firm. These people already have that data.
Some sites use such questions for security (remind you of a lost password) again everything from make and models of cars you’ve owned to mother’s maiden name are compiled for scammers.
3 years ago we joined the biggest HMO on the west coast. Their data mess kept trying to say that I was single and lived in several Oregon/Washington/Idaho cities.
Our 50 year old sons were the ones on my database, listed as me.
We have lived in the same house/address in California for 40 years.
Later, one son joined the same HMO. They showed him still living at home. He had left our home 30 years ago.
The love of his life was listed as her mother, who died 30 years ago, she was supposedly living at the address of her mother’s 30 years ago.