The Eldar (both chocolate and vanilla) should use their superior webway to do raids on humanity everyone- humans can't be everywhere at once, and they can.
Which they kinda already do. Except it's a bit more complicated than that, due to the confusing and twisted nature of the webway. Include the fact that it's infested with demons and Chaos in certain areas, and the opposing Eldar in the others, and you can see that travel isn't 100% safe and effective with it.
Plus the Eldar don't really have the numbers for a full-out war. In the recent lore/codexes, they have been turning the soulstones of dead Eldar into soldiers. Or something like that. Basically borderline necromancy to keep their armies in shape, which is going to get even worse if they constantly raid everyone.
the Necrons. What were they waiting for all these years? Come to think of it, I have no idea what they want now either....
Long story short, the Necrons went into hibernation because they had no choice. The Eldar were much too strong and genocidal, other power psychic races existed, and literal Eldritch Abominations were breaking into the universe and creating their own empires. In other words, if they didn't go to sleep and hide, they would have been wiped out. This was right after the War in Heaven too, so they were in no shape to defend themselves. And the reason why they waited all of these years before waking up is... No idea. I guess the Necron leaders just assumed that if they waited long enough (around 60 thousand years or so) then the Eldar would no longer be a problem and they could wake up and reestablish themselves. Which largely seems to have been the case. Good for them.
The Tyranids, if like the Astronomicon Beacon so much, why aren't they on Terra yet? Space is a big empty thing, for math's sake, it's not like the Imperial fleet can catch them in transit.
The Tyranid fleets were just attracted to the galaxy by the Astronomicon. It was just the sign that said 'Free Buffet'. Their real goal was all the life-filled planets around, ripe for invading.
...
Or something like that. I have no clue, actually, but I doubt that the answer would be much different than what I said. The Tyranids don't seem to set complex goal for themselves.