We can restrict our timeline and location to the New Kingdom Valley of the Kings, but my question is also primarily — how was it that nearly all Egyptian art and hieroglyphics look identical, or rather, what are the differences and 'styles' that we do notice?
How much do we know about how Egyptian carvings, drawings and tomb art was made?
Egyptian hieroglyphs look so regular and standard because that was their exact purpose. There were several scripts for the ancient Egyptian language, as I've outlined before. Hieroglyphs proper are only seen in monuments, tombs, etc, and are thus highly formalized. There are a few instances where we've actually found the rough preliminary "pecniled" designs and grids for wall murals and carvings, specifying the exact sizes of glyphs. There's certainly variation in heiratic, and even cursive hieroglyphs. In fact, there's a whole field of study, paleography, dedicated to investigating ancient handwriting.