Our minds have two very different modes (and a range between). We model important things nearby in more detail than less important things far away. The more nearby aspects we notice in a thing, the more other nearby aspects and relevant detail we assume it has. On the other hand, the more far aspects we see in something, the more other far aspects we assume it has, and the more we reason about it via broad categories and relations.
Since the future is far in time, thinking about it tends to invoke a far mode of thought, which introduces other far mode defaults into our image of the future. And thinking about the far future makes us think especially far. Of course many other considerations influence any particular imagined future, but it can help to understand the assumptions your mind is primed to make about the far future, regardless of whether those assumptions are true.