Very simple really, just a few words. Because it becomes profitable. When you don't have a monopoly of power you have to compete with someone else for it, and so you are predetermined by your position (liable to failure) to strip everyone around you of everything they may happen to have. Almost the same happens when warfare begins (external competition for power), government begins plundering their own population in an effort to retain the power with any means available. When nothing threatens the monopoly of power, it becomes profitable in the long run to turn into "benevolent protectors"
I think no one here idealizes state...
Thanks for your reply. I think I see your point.
Quite a scary conclusion (bolded) you're reaching there. We are reduced to hoping, that the entities in power recognize this and indeed turn into benevolent protectors. Also there are other factors, besides external threats, which might turn them towards looting their citizens, like greed, corruption, lust for power & wealth etc.
If the entities in power don't recognize this (let's assume this) and turn to looting their citizens, they usually wind up as they ended in 1789 in France, as an example. So in the end the power is monopolized by those who are not only able to grab it in the first place, but also able to hold it long enough, which is impossible without becoming a "benevolent protector"