It’s unpopular because it’s a bad argument. It’s not theft because you don’t take anything away. You just create a copy and don’t pay for it, but that’s not theft.
Is it even a theft if I watch publically available unlocked IPTV streams? I mean if they don't want people without paid access to watch them they should protect them with unique logins/passwords and this is valid for whatever IPTV provider (not specific to channels themselves).
It might not be theft but it's not nothing either. Manslaughter isn't murder but someone still died. Copying might not be theft but you're still taking something you didn't pay for.
But it's not piracy either. People just want to make the crime sound worse then "infringement" Might as well call it "software rape" as that crime is closer to what is being done than than theft or piracy.
It is an infringement on one's right to control the reproduction and distribution of their intellectual property.
This right is enforced by the authority that grants it. Viewing, listening, or otherwise 'consuming' this IP is not and cannot be an infringement on these rights. Those who provide are responsible.
If a country does not grant or enforce this right (or on behalf of others) then there is no infringment possible in that jurisdiction. cf. China or Russia.
Moral arguments beyond that are your own and should be clearly segregated from the law. Murder is, almost universally, both criminal and wrong. "Piracy" requires more attention to detail in order to have productive conversations.
A spy steals secrets. Credit can be stolen from you by your boss. Your competitor steals your ideas. In colloquial usage, theft is the act of stealing. The legal term is copyright infringement.
When you "steal" a secret, it's not longer a secret. When you "steal" credit, the original thinker no longer gets credit. In both cases, the thing itself was destroyed: in the former, the secret is no longer a secret at all and in the latter the boss will no longer be considered the mastermind behind the idea. When you "pirate" something the original copy remains and the creator retains it and the rights to sell copies of it and will still benefit from selling copies. It's not theft.