We can't?? Have...have we been trying?
Not the way the Egyptians built them. Their techniques were something different from what ours would be. (
link)
Someone already tried to pick a fight with me about this earlier. He felt like he was really gunning for it too. Basically the last thing I was expecting. So I thought, "Yeah, if I knew he was going to come on
this strongly, I wouldn't have said anything at all because this isn't how I want to spend my New Year's Eve!" It caught me off-guard. So it's not exactly a rabbit-hole I want to go down again.
.
.
.
But my main point was it's not just level of technology, it's also resources available and knowing how to harness those resources to actually get the Spore Drive. Any data the Federation used to have was wiped out. Presumably in 2258. So now they're trying to figure out "How did they do it before?"
In an ideal situation, they would've assigned Stamets off Discovery. But because Discovery can't operate its Spore Drive without Stamets, they had to have him stay there.
The technology for the Spore Drive was invented in the 23rd Century but never perfected. So in the third season, Starfleet had to pick up where it left off 930 years ago but with all the tools rearranged and some of them no longer available.
The key thing that made the Spore Drive work was Tartigrade DNA. But they've established that Tartigrades are now extinct. So that's a major missing piece.