Realistically speaking, why would aliens ever want to come to Earth? We've already harvested a large portion of the planet's resources, and spaceborne aliens could easily find whatever they need in the asteroid belt or mine gasses from the gas giants.

Taking humanity's unnecessary conflicts over the years into consideration, it would be best for a foreign species to not even enter the Solar System at all. If extra-terrestrial beings were to enter Earth's orbit, we'd probably blow them out of the sky.

On top of all that, we also have to take pollution into perspective. If I'm correct, climate change has pretty much ravaged the planet so far, and there's so much trash in the ocean, it's pathetic.

Overall, there could be a number of reason why aliens haven't visited Earth yet. One of them could be that humanity is actually the most technologically advanced species in the galaxy (Which I don't think we are) and other species haven't figured out how to construct spacecraft. Another could be that they're pulling a Star Trek on us and don't want to make contact until we've become space-faring.
Another possibility is that they just don't want to visit our planet or they simply don't exist at all.
Another one is that they DO exist, but are too far away.

The possibilities are almost endless.