Could someone please explain to me why it’s bad to share family recipes?

I don’t get it.

Context:

I asked my mom for a family recipe. We ate it every year on Christmas Eve growing up. I enjoyed it a lot. I hadn’t had it for 5 years since I’ve gone plant based but I think I know how to make it whole foods plant based. I understand the value of this recipe, for me and other members of the family, when it comes to sentiment.

But after my mom sends me the recipe, she says: “I want to make sure you know this is a family recipe; don’t give it away to other people.”

And I wasn’t planning on giving it away (nor have I). But I don’t understand the big deal. So I told my mom this. “Ok. I wasn’t planning on it, but why can’t family recipes be given out?”

What followed was 1/2 an hour of “you just can’t / grandma asked me not to/because it’s a family recipe” and me saying “I’m not going to but can you explain why.”

Now my mom has stopped responding and I think she might be upset with me. But I still don’t understand because it’s sentimental, yes, but it’s not like allowing someone else to have the recipe would take it away from me or my family. It’s not like sharing it would degrade it in any way. Isn’t it nice to share things that make people happy? Can someone please explain?