Some half-cooked answers I've found are that, by the time they had interest in the region (oil in the 1920's, moreso in the 30's) coloniazation was frowned upon. but the King Saud at the time was the king of a desert, with nothing in the way of an army by and stretch of the imagination?
So instead of royalties and generally caring about what the Saudi's thought, why didn't COSAC/ARAMCO/USA/UK just take power, it would have made things simpler, no?
I've been reading a lot of Saudi and the birth of their country, and noone has approached the subject.
You've pretty much answered your own question. Invading and controlling countries was not the preferred method ot at least was losing favour. That doesnt mean that some control was entirely off the books (vis a vis Persia and the Brits), but that it wasnt the ideal way of gettings one way. It was clear that a pro-western government could be dealt with via treaties, bribes and business arrangements rather than unpleasant invasions and occupation. The fact that Saudi Arabia has remained western aligned and at least partly benign for 80 years odd is evidence it was a reasonable plan. British expansionism had largely stopped at this time and was indeed receeding.
Additionally the US had never, as OlfactoriusRex suggest, ventured too far into interfering directly in these regions, partly as a matter of policy but also as a matter of practicality. US power projection capabilities at that range were fairly poor until post WW2. Some military bods would know more on that than I. The UK of course was the most expert nation at that sort of thing around this time.
The question is thus not so much why didnt they, but why would they?
Just finished reading Daniel Yergin's "The Prize," and it goes into a very oil-slanted history of the kingdom. In short (and I'm sure others here can add more), after WWII, the British empire was receding and the Brits saw a large presence in the Gulf as unsustainable. The US, at least under Truman, saw direct involvement in the Gulf states as "a little far afield" of the nation's policy and instead encouraged private business to take the lead, especially in getting energy to Europe in a somewhat circuitous application of the Marshall Plan.
Seriously I suggest you look at Yergin's "The Prize," it's an imposing tome but you could skip to the mid- and post-WWII stuff focused on Saudi Arabia and look at the fading British presence in the gulf and the rise of American influence (specifically, corporate oil influence) in the region.
Overall Yergin's story is incredibly fascinating in how the locus of power in the oil industry swings into the Middle East, and very much into Saudi Arabia (and Iran), in the last 30-40 years. It's really interesting and gives a perhaps unique perspective on the region's history through the lens of oil.