It is a really good point, and one that I don't reallly understand either.

Because surely if the paintings captured their personality too, almost like a brain, even when the person was dead, then everyone would get paintings done. Like if there was a painting of Sirius would Harry be able to talk to it and ask it questions? It would seem so easy then. But the Phineas painting was quite real and had opinions and everything, very much involved in the real world, helping Dumbledore.

It's almost too good to be true. Maybe paintings like that are rarer, or harder to do. I think you're right, the only person who understands is JK. And maybe she didn't want us to think about it this much. I really don't know.