Why are dandelions considered weeds when it's an edible flower herb?

Because they take over pasture, vegetable patches and gardens so rapidly that it becomes impossible to control - they outcompete the desired plants.
Secondly, there is a plant called false dandelion or flatweed which looks very similar. Over a three months, it is poisonous to horses and causes a neurological condition called staggers, gradually paralysing the hind legs. There is no cure. Euthanasia is the only way to prevent a death of great suffering.

I think they are pretty, like tiny little sunflowers.

They grow when they're not asked - that's the definition of a weed.

They grow everywhere when people don't want them to or maybe places they aren't native to. I suppose that's what qualifies something as a weed.