We assume all of that.

We really don't know how common objects are between stars. We've observed a couple of interstellar visitors passing through our system, old asteroids from origins unknown. But we can only track the really large ones. We can see mid sized objects when they come very close to earth, but we can't track small objects at all, we literally have no idea how much of that exists in deep space. (Or for the most part, in our own solar system)

Still, you can see just from the evidence that our deep space probes have never really (hit) anything, that it would in fact be a quite rare event. It's just hard to say how rare.

aaaaand that's why we will NEVER reach 99% c

Dust at that speed would be CATASTROPHIC.

We just need to discover astrophage.

See The Expanse if you want to see this in action at speeds significantly less than c.

Edit: Even for just getting around in the solar system, they had to magic up some chemicals to allow the human body to withstand deceleratory g-forces strong enough to keep travel times short enough to not completely wreck the narrative.

They do slow down after a bit:

Once nearing the ground, drop pods activate a drag chute (more formally termed a "Drogue Panel"[10]) formed from the upper-most panels. The chute deploys and begins to slow down the pod's descent. Once about 1 kilometre (0.62 mi) from the ground, a series of computer-controlled retrothrusters engage and rapidly decelerate the pod such that it can impact safely.

Also most (not all) drop pod deployments are used by specialized troop like ODST or spartans:

Of these, the UNSC are particularly reliant on the use of drop pods thanks to the aptly-named Orbital Drop Shock Troopers; specialised shock troops who specialise in the use of drop pods to insert behind enemy lines and conduct special missions.[7][8] Although the ODSTs are the primary users of drop pod insertions in the UNSC, both Spartans and regular Marine forces may also use them.[9]

https://www.halopedia.org/Drop_pod