I am someone who does a lot of things many people feel are hard. I solder SMD on component level, I play fretless instruments, I program. Yet the hard part is never what you can just imitate.
Soldering SMD is something I can teach a non-electronic person in a few hours, for example. The hard part is knowing what to solder and when. Similarily learning to do the pure technical act of drawing or painting photorealistically can be learned. To paint motifs that are worth doing so and result in a "good picture" are something different entirely.
Maybe you have to take my word for it, as someone who in his life went through hundreds of art school applications, but there will be a difference between some abstract thing a person with true skill and talent did, and something your five year old neice did (nothing against her, maybe she has talent, who knows). Something being (seemingly, to the layperson) easy to copy does not mean that it is. Very often there will be much more behind the work than it seems on first glance. A lot of the very abstract stuff you will find in the museums for example was done in a time where nearly nobody dared to do this. So if your neice wanted to copy that, she needed to go back in time, grow up there and do it back when nobody did it. So yeah she could maybe do it. But she didn't.
When comparing abstract and concrete painting there is a lot to be discussed, but trust me when I say that people in the art scene probably look at art just in a different way that most people. Many people have misconceptions about art a la "what does the artist want to tell us" and then there is just a bunch of splashes on the canvas. The artist can sometimes be a trickster and the art understood from the context in which it was created. Sometimes art also just is and in a world where everything has to be about money, efficiency, entertainment, etc it can serve as a room of reflection with something that is beyond simple answers. I am not going to give a grand defense of abstract art here, I am not a painter and if I were it would probably not be purely abstract.
Now historically painters went into the abstraction when photography came around, which could create the photorealistic image very easily. Nowadays photorealistic painters often just take a photograph and (like your neice), copy it to the pixel. That is of course a lot of work, and maybe there is even a bit of art or rebellion in that amount of work itself, but I find it more interesting when people pull out things from seemingly nowhere in their very own and unique ways.
But I am a musician and I work a lot with harsh noises many people hate to hear. I don't think there is any need for me to even attempt to defend what I like to do and so shouldn't painters.