The Manhattan Project had nearly unlimited resources at a time of scarcity, and it took enormous engineering risks. The risk component is something easy to overlook, but these were enormous infrastructure projects by any standard done at scale without much chance to prove them, and they were accomplished in incredibly short time-frames.

Could you imagine someone building a full-scale, power generating fusion reactor off a new and untested design in a about a year? More to the point, could you imagine someone funding that?