Because Grandma had her mom watch her kids so didn't spend $14,400/year on infant care, and her mom had time because she got fired from her school teacher job the day she got married!

Today's mom, on the other hand, moved to San Francisco to get her data scientist/devops/whatever job (there were not any in her area of northern Minnesota and she kind of liked CA after going to Stanford) and Grandma doesn't want to move to CA because it's going to fall into the ocean, you know.

Caring for an infant is hard, high-skilled work and something you want to get right. "Shaken baby" is a real thing and can lead to development disorders. A hands-on attentive caretaker has a very good ROI -- healthier, more communicative toddler and child. "You get what you pay for [or do yourself]" is a real thing for the first few years of a child's life, and having family nearby, which we fortunately did, was invaluable.