I've run myself in circles thinking about this, so here's a wall of text.
First assumption - Wizards would be able to (openly) rule the world if they wanted to
Okay, I guess I can see how this might have been true in the middle ages when someone basically being a walking Howitzer was totally OP. But now we have Howitzers, and things better than them, so they aren't more powerful. One mage might be better at being "all-knowing" if he has scrying spells but now we have military satellites, cell phones, GPS and more. Okay, so one modern human + gadgets is still potentially better than one mage. If your mages can live a really long time, I guess he might be wiser than a normal human, but wisdom only counts for so much. So...unless your wizards are crazy OP, modern technology should provide a decent challenge for them to keep up with. Given your description at the top, I'd assume a wizard to have power equivalent to two or three Spec Ops teams which would be enough to inconvenience a country but not to hang onto a country.
Second assumption - Wizards want to rule the world
Alright, people with power maybe want more power. But if they can't be a dictator in a country, there might not be much in it for them, as politicians in a democratic country tend to have relatively little power by themselves. It seems like maybe they'd be better off starting a cult if they wanted to control people. Maybe some do?
Third assumption - Other wizards would care
So...say a wizard does take over a country or something. Do other wizards care? If that person interferes with their goals, probably. If we make the assumption that institutionalized faith organizations have some branch that deals with wizards, they would probably be against a wizard taking over a country and then preventing them from operating there. Maybe their magic is more/less powerful depending on the strength of belief in their religion so losing a chunk of believers is a considerable hit. Okay, so now we've established that large magical organizations probably care and individuals might care.
Assuming there are magic organizations, they likely have people in positions of power, or at least connections with them, in order to push for the things that matter to them by working within the system.
It also seems possible that dictators of small countries could be mages (or shamans, w/e) that use magic in less obvious ways to divert the occasional bullet or something and make themselves into powerful warlords without having to directly confront everyone that challenges them.
Others might create living cults (if they use ritual magic), some maybe death cults (binding the souls of the dead cultists to use their faith power eternally) while a bunch of others may rely on natural forces (ley lines and such). Clerics and priests may tap into the power of faith.
TL;DR Overall, I guess I would say that mages wouldn't rule the world because hanging onto something like that is hard, so aside from an occasional nutcase they probably have better things to focus on. These 'better things' would mostly relate to where their power comes from.