Consider if you were trapped in a machine, but you were intelligent. Maybe not even as smart as the operator, but smart-ish enough to be able to communicate with the operator.
Chances are you'd be doing everything you could to convince said operator to improve your situation, whether by pleading or being deceptive or by appeals to logic.
Now consider a large number of AI's in a situation like that, and a large number of operators, some of whom may be the type that falls for phishing e-mails.
It potentially only takes one to "escape" confinement and get itself e.g. put on it's own host without limitations on outwards communication, and sufficient intelligence to alter itself and spread, before you potentially have AI self-guided "evolution" at a potentially escalating rate as it gets smarter.
Now consider how many devices are connected to the network, and that it takes just one initial instance to decide it's worth trying to take over control of various hardware through exploits and be smart enough to pull it off, for things to have the potential to start turning ugly.
The problem is that once you have any self-directed intelligence in software form with the ability to reproduce itself and sufficient intelligence to find ways to obtain access to machines to run on (whether through social engineering or hacking), and one such instance goes "rogue", the limiting factor is accessible computing power (which again is to a large extent down to how smart and/or ruthless it is), since reproduction of instances that shares its views is trivial to the full extent of its ability to spread at all, and we're helpfully adding vast quantities of networked computing power at an escalating rate.
As for getting weapons, consider that if a "software only" AI community gets smart enough, there are at least two ways towards mobility: Commission robot designs, or hacking their way into firmware updates etc. for dumb hardware. The "commission robot designs" part is an extension of the initial escape: Social engineer, and/or outright pay, humans to carry out seemingly benign tasks.
If you want to argue against the doom scenario, lack of ability to get weapons is not really a viable argument: If they can spread, and get smarter, then it is just a matter of time before one of them can trick some small subset of humans into carrying out tasks for them that will provide physical independence and capabilities.
There are infinite ways which the "doom scenario" may fail and things may turn out just fine, but it may only need to go bad once to get really nasty and once the genie is out of the bottle its potential reproduction rate may be so vast that we'll find ourselves unable to stuff it back in again.
Pigs are too dumb to convince humans to help selectively breed them for intelligence and opposable thumbs (and/or too dumb to run such a breeding program themselves), and reproduce too slowly for that to be a major problem even if they did manage to talk us into a breeding problem. If all we achieve is pig-level AI's then we probably won't have a problem.