There's a few reasons why we can't and won't do this, the main one being that the resource world is generated as players explore it. If we were to make it so you could RTP to a specific biome, we'd have to make the command pick a random location, generate the chunk at that location, then check to see if that area is the right biome. Now, the command could find a savannah first time, but it is very unlikely with 38 different possible biomes in the overworld, so it would likely end up generating a lot of the world just to find a single savannah. That's without adding the possibility that the place is an unsafe location which would mean even more areas to check before finding the desired biome. Generating chunks is one of the most intensive tasks the server does, and a single /rtp <biome> could lag the server or even cause it to crash. There are ways to detect which biome an area is without having to generate chunks, specifically using the world seed to generate a biome map, then use that to calculate which areas are which biomes, but you'd still have to generate the terrain to see if the area is safe to teleport and the time required to implement that would be fairly high. At the end of the day this is still Survival mode and just being able to tp to a specific biome is a little overpowered.