Why are parents so worried about my social security number?

There’s a good chance I could be naive, so bear with me. I’m 19 and I’m starting all my adult stuff, including having my social security number memorized. When I told my mom I had it written in a locked note on my phone, she freaked out. I told her it needed a passcode to find it, but she was still upset. She didn’t really give a good reason either. Is it because of identity theft? I have my number memorized now so I don’t have it written anywhere, but I’m just wondering if this is a generational thing (my mom is in her late 50’s)or a valid worry