It is intrinsic to the design.
The open nature of the protocol carries with it an implicit trust. Any client may join and speak to any other client and be heard. It is a social contract, just as we have "in real life."
Imagine someone knocks on your door. You can choose to not answer, but even considering the choice costs you cognitive bandwidth. As long as you're within earshot of your door, a knock will cost you attention.
We have solutions all across the spectrum, from putting up a "no soliciting" sign, to arresting bad actors. But what if you have a riot outside your front door (a DDoS)?