Building helium 3 mines off planet are massively expensive, especially considering that we don’t have any colonies in space, you literally have to build everything from scratch. If we had colonies in space already, then maybe. This means that anyone that wants to do this will pay every high bill for something that there MIGHT be a market for in some future, its a mighty big risk for the investment needed.

In addition to that, most Tokamak reactors don’t use helium 3, so there is no real demand for it.

Apparently there is a way to synthesize Helium 3 on the earth, and it obviously would be cheaper than building colonies off planet.