Making a client is a lot harder than making a server. Programmers are willing to work for free but artists don't. The artists and world builders need a unified vision for a game world so it's less likely that some anonymous artist will drop in an asset to use in the same way a programmer can just commit to Github. Most of the code is rather boilerplate and game agnostic so the solutions were invented elsewhere and just need to be implemented.
It's not just the models and world geometry either. Consider the amount of labor involved to make all of EQ's items and crafting recipes. Emus just have to scrape this. TAKP has over 25 thousand item rows. One row is 154 columns, but not all fields need to be filled by hand.
It's simply a tiny fraction of the amount of work to make something like THJ than it is to make a game from scratch, even if using game engines. If the goal is to recreate a game then the emus just have to do it once then all the others can copy it. THJ leveraged two decades of work. The end goal being a recreation nearly eliminates any disagreement on how it should be built or what should go into it. If creating a game from scratch, there is no obvious vision or end state and people are much less likely to contribute for free to work on somebody else's game. I used to fight with the other TAKP devs a lot even with the shared goal of recreating AK.