It doesn't always happen this way. When forecasting solar eclipses, the sun blowing up, or the heat death of the universe, scientists are prepared to step up. The problems seem to arise when predictions are difficult, or there is uncertainty. Of course these are the areas where the best input of scientists would be most valuable.
Instead of scientists, technical experts seem most attracted to futurism. There, taking a long term view sometimes seems to have some associated status, and people are not quite so reluctant to look to the future. Futurists are a bit of a motley crew, though. They are not necessarily folk which scientists gain by affiliating with. I suspect that that's a big part of the problem.
I think more evolution enthusiasts should step up to the plate. Now we have a reasonable stab at a science of cultural evolution, we are better equipped to look to the future of evolution and consider its possible attributes. Scientists not expressing their opinions or not studying the topic is problematical. It leaves civilization without very much foresight - and without the ability to see, it becomes harder to steer.
One popular meme which recommends avoiding long term forecasting places a so-called "singularity" in the near future - and claims that making forecasts beyond this is practically worthless. This meme is hokum. Machine superintelligence might represent a significant change, but it is not one that makes all of our models fall to pieces. I think that making this claim regarding the impossibility of forecasting shows naïveté about models and forecasting in general.
Most of those involved in long term future forecasting are not very well versed in evolutionary theory. Indeed, one of the popular ideas is that evolutionary theory isn't going to be very relevant because natural selection is horrible and cruel and so humans are going to decommission it and go off in their own direction. This is a poor excuse for not making more use of evolutionary theory, IMO.
We are, by most accounts, in the midst of a major evolutionary transition. Evolutionary theory knows some things about major evolutionary transitions. IMO, it's time for more scientists to step up to the plate and share their best forecasts.