> It's not asking why they haven't contacted us, it's asking why we can't see them

Space is vast. The strength of communication signals drops off quickly with distance. All life is subject to the laws of thermodynamics.

The Fermi paradox was proposed in 1950 during a time of rapid growth and empire. At that time, radio communications were powerful, and it seemed like they would only get more powerful as our civilization advanced. But that's not what happened. The key to better communication is not more power, but better noise reduction. Since WW2, our radio signals have become weaker and weaker and our receivers smaller as we transmit information more efficiently. Ideally we would transmit signals with the bare minimum amount of energy needed for them to be reliably received by the intended receiver. Any civilization that wants to not waste energy (read all of them) would do the same.

Because power drops off with distance, a more sensitive receiver is required to receive a message at a greater distance than the one intended, and this required improvement to sensitivity increases rapidly with greater distance.

Your cellphone communicates with an antenna approximately 2 meters in length with a maximum range of about 70 kilometers. An antenna of the same technological capability able to pick up your cellphone signal at 1 lightyear would need to be 2900 km in length. At 100 lightyears, you would need an antenna over 37,000 km long.

If you can bounce a signal off satellites, you can communicate with any point on earth - thus the most powerful radio transmissions we need to send for our own utility need to be detectable by a 4 meter dish 30000 km away in geostationary orbit. To detect such a transmission at 1 lightyear requires a receiver 203 km in diameter, at 100 lightyears a dish 2600 km in diameter. Beyond 2000 light years, you need a dish larger than Earth. If you need to communicate any further away than that, you're using highly directional beamed communication between planets which won't be detectable unless the receiver just happens to also be along that beam line.

Thus to detect a radio signal from another civilization using a telescope we could conceivably fit on our planet, one of the following must be true: the transmission must be from a relatively nearby world (0.16% of the milky way in range if they are our technological peers, less if they are more advanced than us), the transmission must use orders of magnitude more power than is necessary for communication (either they are using dramatically less advanced technology or intentional broadcast), or we need to detect highly focused beamed communication (either we get incredibly lucky or its an intentional message directed at us). It would be presumptuous to assume that aliens would set up a colony conveniently close to us, that in our mere 100 years of radio use have already greatly surpassed them, that they have maintained an active beacon for long enough that we would happen to see it, or that they would be trying to communicate with us specifically (especially given that any civilization more than a few dozen light years away would have no way of knowing that there is intelligent life on Earth.

There are of course other means of detecting life besides radio communication. Perhaps we may see pollutants in another world's atmosphere - but we have only directly detected a few thousand exoplanets thus far and observed the atmospheres of only a tiny fraction of those. Perhaps we could detect megastructures like dyson spheres - but to know what we are looking for requires a lot of assumptions about how they are built and we don't even know if they can be built. Really any sign of intelligence detectable over interstellar distances other than radio waves is purely speculative.

The assumption that evidence should be commonplace is poorly founded. Unless the universe were teeming with civilizations at a similar technology level to our own, we shouldn't expect to see anything at all.