I guess I still just don't really see it... like, coal plants are also much less energy-dense than nuclear fission. So is solar, so is wind. We build all of those anyway. There are lots of things other than power density that contribute to whether or not a particular generation technology is economical.
As to why massive fission reactors aren't built: there are plenty of already-available passively-safe/meltdown-proof fission designs (many gen-IV designs qualify), and from what I can tell, the reasons they're not built are as much political as anything -- people don't like them, and the consequent regulatory regime has made any fission projects prohibitively expensive regardless of their size. None of this need be the case with fusion.
As to tritium: I think you're overstating the tritium risk. They're only dealing with grams at a time, and even if it all leaked out, it would rapidly diffuse such that risk to the public would be infinitesimal as compared to normal background radiation (plus its half-life is only something like 12 years). ITER has a safety page: https://www.iter.org/mach/safety that essentially says as much.
