I can think of two possibilities:
Are you playing a world of a previous version (1.15.2 and below)? If that's the case, then you're most likely going through chunks generated of older versions, where none of the new content can be seen. You need to go further to unexplored areas, to generate new chunks and will result in generating new biomes with the new mobs, items and blocks.
Have you updated the game? Also could be reason why none of the content aren't listed in creative.
