Let's walk through the security systems stopping each scenario one-by-one:

> It could scale it's cognitive ability very quickly by taking control of a lot of compute all around the World

...if it had access to an unlimited amount of idle compute. That requires either buying it (and AIs don't have a meaningful allowance) or hacking into a VPS with a novel breach-of-access attack. The latter scenario is entirely infeasible; but I will give you the benefit of the doubt and assume that our hypothetical AI was given a non-insignificant amount of VPS space by it's creator.

> and then do cyber attacks with pace never seen before.

Like Stuxnet, the one that humans wrote? Or closer to the Chinese/American infrastructure threats? There are a lot of cyberattacks that happen even on a daily basis, it would take something truly unfathomable (eg. it hacked US BLUFOR datalink) to stand out from the ordinary. Even then, the actually dangerous weapons require authorization that an AI can't provide. The best route around that would be social engineering, a "hack" best performed by real humans and not a disembodied language model.

> It could easily earn vast amounts of finances by blackmail, ransomware and other actions.

Hey, maybe so. Even still, humans pioneered both of those and it's not a novel attack coming from AI. Likely, but also not very different from the status-quo of call centers and automated scamming.

> It could then recruit humans as proxies using the resources gained (blackmail, money) to create a physical presence.

To an extent. How do you resist adversaries, like the police and the government? Supposedly your AI has an advanced online presence to be able to recruit other humans. Where is the money stored, how do you hide it from the feds and convert it to fiat when you pay the proxies? How do you hide your communications from surveillance? Worst of all, how can you trust double-crossing meatbags that lack the calculated certainty of a computer program? What's to stop them from taking the AI's money and selling them out?

> It would then use this presence to get access to physical infras, weapons, etc until it has factories setup where it can manufacture robots, drones loaded with its software.

It would be lucky to get as far as two country bumpkins holed up in a barn with their "talking computer friend". You have a very active imagination, but literally every single example you have posed so far is a human crime that the police has tactics to mitigate. You are not going to funnel millions of dollars to an AI because the finance system is designed to track fraud. You won't recruit dumb people online because the US has intelligence agency employees literally hired to infiltrate these recruitment schemes and destroy them from the inside. You won't buy a factory for manufacturing artillery and bombing drones because the local Chamber of Commerce wants to tour the facility and you have until Friday to figure things out.

This is the reason why imagined AI threats are not being taken seriously at-scale. There is a danger that these threats are emboldened by the help of AI, but even assuming AI attains superhuman traits, it's not going to get around the inherent limitations of the human and the justice system imposed by society. The advent of the internet meant anyone on the planet could share weapons schematics with anyone else, wherever they are. The overwhelming majority of internet users never do anything more advanced than pirate an episode of The Simpsons; I would be shocked if AI turned out any different.