[DustyPorViva]

Pixel art = attention to every pixel, control of every pixel. When you use an airbrush tool, for instance, it lays hundreds of pixels(depending on the size) of various colors, size, and in different places--none of which did you place directly yourself. You had no control of how many colors it added, where it placed them, and how they directly effected the pixels around it.

Layers are debatable. If you place a layer of a window over a house and make it partially transparent, you're going to add a lot of colors depending on what it's placed over. Many people don't have a problem with this, however, as you are placing pixel art over pixel art, so the color addition would be minimal. If you don't, most people end up manually emulating the same thing anyhow. Manual process is more efficient when you're trying to keep color-count low, though.

Either way, it doesn't depend on the program you use, but how you use it. Photoshop is capable of pixel-art, it's just not tailored to it.