1.b. Space faring civilizations probably don't live on planets the same way we no longer live in caves and mud huts, they build modern houses and cities out in space near to their preferred energy sources and transportation networks. It's a lot harder to hit something smaller and more mobile than a planet and even if you do it doesn't have the same consequences that it would on the ground. i.e. more likely to just punch a big hole rather than convert all its energy to heat, no pressure wave, your neighbours are further away, etc.
2. This is probably why we don't hear anyone, either they are smart about their use of EM communication and everything high powered is directional or they are even smarter than us and have figured out some form of FTL communication.
Whatever protection system one comes with, it would better be good 100% of the time. Because a 99.999999% success rate will be a death sentence.
> It's a lot harder to hit something smaller and more mobile
People are very inventive when it comes to new weapons. By the time we have the technology to reach 0.01c, we'll have invented 10 more generations of weapons. We might miraculously avoid interplanetary wars before we reach for other stars, but I would not bet my money on that.
1. Interstellar travel is genuinely hard, so hard that advanced civilizations do it from time to time, but not so often to result in "percolation". In order to have a reasonable argument, let's start from contemplating how hard it would be for us, humans. This is the only civilization we know of at this point. There are two things that make interstellar travel very difficult: 1.a. potential collisions with dust grains and 1.b. violence.
1.a. Danger of collisions. According to wikipedia, cosmic dust grains can have a size up to 500 microns. At a typical density of 2g/cm3 this is a mass of 0.5 mg. Assuming you have a spaceship that travels of 0.01 the speed of light, the energy of impact with such a dust grain will be about 20% the energy of an armor piercing round shot from a modern main battle tank such as the M1 Abrams. Over a 4 light year travel (the distance from us to the closest stars), or a few hundred years of flight at 0.01c, there would be billions- trillions of such impacts.
1.b. Human violence. Assuming we get the technological prowess to accelerate things to 0.01c, what's to stop someone to point a little projectile in the direction of Earth, or some other place colonized by humans? Such of projectile weighing only 15 kg would have the energy of the Hiroshima bomb. In fact, it's actually most likely that the road to interstellar travel will start from military applications, just like the Apollo program was a side-effect of the arms race that created the ICBMs.
2. Interstellar communication is hard. Let's say that aliens are indeed everywhere, including in the planetary systems of our closest galactic neighbors. How would we know? Would SETI find out about that? The only way we'd find out were if they were pointing some large antennas straight at us, and putting some megawatts of power into beaming signals at us. How do we know that? We know because we get radio signals from Voyager 1, which is 150 AU from us. Proxima Centauri is 1700 times further, so the signal would be more than 3 million times weaker. Voyager's antenna is 3.7 meters and the transmitter has 22 W of power. So, if our neighbors are not actively trying to beam signals at us, our chance of detecting them is practically nil.