I think it is a big mistake to assume that humans are the only intelligent species on earth. I think there are plenty of intelligent species, maybe even smarter than ourselves (at least in certain aspects), here in the planet, but what we forget to take into account is that there are many other factors, other than our intelligence, that have lead to our technological advancement. For example, simple things like the fact that we walk on two legs and have two hands, which are extremely versatile tools, free for use (dolphins, for example can't do much with their fins, even if they are pretty smart). We use our hands as tools to create other tools, that in turn help make other tasks easier and create even more specialized tools. Another factor that is crucial is the time that we get to live. We are fortunate in the fact that we get to live up to 100 years or more. Octopuses, for example are extremely smart and good problem solvers, and they have tentacles that are very versatile tools, but they only live 3 to 5 years, which is a very short time. On top of that, there is yet another factor on our favor and it is the fact that we are a social species that learns from other individuals in the group. Again to bring the example of octopuses, they live mainly solitary lives and die when they give birth, or after sex in the case of the males, so even though they learn things very quickly, they don't have the ability to teach their offspring anything that they have learned so the newborn babies have to relearn everything by themselves all over again throughout their lifespan. So no, intelligence is not really THAT rare, it is the accumulation of other factors like the ones I've mentioned, and maybe others that when put together have made it possible for us to advance to our current technological progression.