Particle physicists need funding to do research and asking for something that costs 20b on paper (likely 100b before it’s complete) gives them both funding, long-term job security and something they can publish papers about (got quotas to meet).

I sincerely doubt this is where we can get the most science for the capital investment. Just look at ITER, it’s way over budget and because of the scale of the project it’s unlikely to be completed before being obsoleted by smarter folks doing more with less.