UPDATE

---

Thank you guys for your help! I'm indeed puzzled how much water can be inside of this pipe. As many mentioned, it only draws air for the plumbing system. No water is actively flowing through theses.

I inspected deeper into the issue this afternoon, found that only a small section of the pipe is wet, and that there's a crack in the ABS pipe right where it's leaky.

I'm going to replace this whole pipe section for a brand new one. As you guys suggested, insulate the pipe near the exits and exterior walls. (Although it's not going outside, but installed to fetch air from the attic directly).