In addition to the hundreds of blocks and mobs that would need to be implemented properly and rarely are, the lack of mod support is a killer.
The only "complete" reimplementation of Java Minecraft that I'm aware of is Bedrock.
Far from it. The versions lack a lot of parity and Bedrock is called "bugrock" by the community for a reason
I edited in scare quotes for "complete" to make that clearer, but I mean in terms of at least having matching blocks/mobs despite many differing details.
A lot of the parity issues are due to Bedrock not reimplementing bugs from Java (quasi-connectivity aka "droppers are doors".)