Dark Age of Technology humanity did have at least one war with the Eldar. It is stated that during the war, humanity moved the entire Sol system through the warp and placed it right on the doorstep of the Eldar Empire as a threat. The Eldar immediately backed off and retreated to their core worlds (the ones now in the Eye of Terror). That is canon fluff. The Eldar viewed themselves as masters of the galaxy but only occupied a tiny corner of it. When humanity rose to power, it spread throughout the galaxy as a whole and crushed anything that attacked it.
The Eldar "co-exist" with no one. Besides, before the Fall, there were no Farseers. The ones who became the first Farseers were those few who foresaw the Fall and convinced others to follow them and to construct the Craftworlds. After the Fall, Asuyan trained the first Aspect Warriors.
After the Fall, the Eldar were amongst the first to raid human worlds. Admittedly, this was out of desperation. Currently, the Eldar seem to be in a state of "blissful" ignorance. They honestly believe they are dominant and that everyone else are squatters. Would you want to be friends with someone like that?
Furthermore, the Eldar never actually advanced at all. They are explicitly stated to be using technology and sorcery/magic/psychics/whatever that were given to them by the Old Ones. This bit them in the butt after the Fall because they never had scientist or philosophers or intellectuals. Everything they wanted was given to them by the technology the Old Ones had given them. They never even evolved, having been created by the Old Ones, given their technology and sorcery, and unleashed upon the Necrons during the War in Heaven. They never even discovered fire. Kinda sad, really. They will never be able to have pride in any sort of achievements or advancements or discoveries because they will never have made any.
In comparison, by the height of human advancement, humanity had (canonically) all but matched the Eldar through sheer technological might. Though Eldar technology was said to be more sophisticated. That is like the difference between an iPhone and a Droid smartphone. Where the Old Ones tech the Eldar used (probably a lesser version than the originals, for fear of rebellion or something) often combined science with magic to create an unbelievably advanced piece of technology, humanity achieved about the same result through scientific knowledge alone. Then, just before the Fall, the first human psykers (excluding the Emperor) appeared and began developing and studying their powers. Human psykers are said in the lore to be more powerful than their Eldar counterparts. All Eldar are psykers, but the most that the vast majority can do is mild telepathy and empathy. The weakest human psyker is fully capable of ripping your mind apart or your soul out of your body. I would imagine that Eldar hubris would have eventually lead to another war to determine who was "superior" or some other pointless thing if the Fall had not happened.
As it is, in one of the Eldar novels (though if you take it for canon or not is up to you), an Eldar Farseer of Alaitoc is shocked to discover (the hard way) that Imperial ships have void shields. She was one of her Craftworld's leaders and was so removed from reality that she didn't even know her Craftworld's primary enemy had shields. That shows a distinct refusal to accept reality on her part and, given her station in Alaitoc, likely the entire Craftworld. Possibly her whole species.
Basically, the Eldar "let" humanity become so powerful pre-Fall because they rejected the fact that someone could be as strong as them. Then got their rears handed to them by a new species and went to party it off, got too high, gave birth to an eldritch abomination that thinks it's a god, got eaten by said abomination, and then they had no power to stop the storm they had unleashed upon the galaxy (not that they would if they could).
The Eldar would never have expected a species to advance so quickly. To them, they glanced away for a moment to do something else and when they look back, we've gone from stone age to overwhelming. From one world to everywhere the Eldar look. All on our own merits. Unfortunately, this is also what grabbed Chaos's attention and focused it on us. That didn't end well. Then we were the only species devastated by Chaos that managed to crawl back to the top. That had the unfortunate effect of keeping Chaos's attention on us.