I hope you agree, that successfully preventing spam does not depend on it being impossible? When it is costly already it will be less of a problem, than when it is more dificult.
And since every ongoing conversation on servers like those from Signal or Simplex will use some of the providers resources, spamprevention is important.
Every Bot can create Simplex accounts and text over it without restrictions.
With signal at least someone needs to register a phone number. This is a hurdle. And this hurdle probably decreases the number of non-human users. Which is the purpose.
Since Signal wants to offer privacy to human. Not bots.
