There's a reason for that, the same reason why new biomes aren't added to already existing worlds. Blocks would be forced to be replaced, deleted, and added, and it would most likely wreck builds, mines, etc, and cause glitched biomes. For example, let's say there was a taiga biome in an already existing world, but the newest biome took place in the same exact spot. There would be leaves and logs floating, foxes in a savannah area, etc. This was intended on purpose, probably since players would be extremely angry if something they had been working on was ruined.