Because I did nothing. Well, I did something, but that something took about six seconds.
I put the ends of my store-bought green onions in a jar of water. Then I set it on my kitchen windowsill.
And guess what? Just like on the pinterest board, my green onion stumps grew back! Regrowth started immediately, and we were using the onions within a week.
I haven't bought green onions since - we just snip off what we need with kitchen shears.
Every so often I change the water and rinse the roots.
Rocket science, isn't it? Well, you people are smart. I'm sure you can manage.