You're correct that evolution can happen in a much shorter time frame. However, we are heavily specialized, multi-cellular type organisms and any slight change in our DNA chains are minimal compared to the evolution a species of bacteria or even fungus can have. The more complex the being, the slower the evolution will be.
I do believe that 'races' as we see them in humans exist only in the context of subspecies and you are correct that it is often difficult for ethnicity X to interbreed successfully with ethnicity Y, and this act is not without its challenges (see blood marrow donation issues for mulattoes on this) but I also think we were not meant to fly or create devices with supersonic speeds or instantaneous communication devices. We did all of these things, somehow, and instead of wanting to use our non-negligible resources at our disposal to improve the lot of the lives of other races/ethnicities around the world in their homelands, we think it charitable to invite them to ours instead.