In Doom 3, it's established that the UAC is pretty much in charge of... almost everything. They no doubt control the entire Earth. Government, nations, it's all under one corporation and there's probably little to nothing on the planet they don't control. They have powers that go beyond any law, and infinite funds.We do know there is some outside activity going on they can't control(Martian Buddy, other spam companies that operate as small businesses, but for the most part, the UAC is top dog and competition probably only exist because the UAC allows harmless competitors to stay afloat).
That's one point- Swann shows up because the UAC is mad that Betruger's experiments have been costing so much money. True, the odd events have also been cause for concern, but the money? What does the UAC care? They're implied to print money as they need it and probably have the authority to say "no inflation".
Second point- the UAC controls everything, including the military. And yet Mars base, the primary off-world research facility where very serious things are done... is guarded by a small detachment of men and the top rank officer in the whole of Mars is a sergeant? I know they're probably not expecting outside thieves or terrorists, it is Mars and any transport ship would be detected long before it got there. Internal theft and conflicts are probably rather rare. But still... they dig into the unknown and were doing experiments in dimensional transport. They do various research. At the very least, they should have had a lieutenant in charge with sergeants in charge of a squad in each section of the base, not putting one disgruntled, bored sergeant in command of a handful of people for the entire Mars City complex. Even if they don't want armed soldiers everywhere, making people nervous, they could have just set up a Marine base on Mars so they'd always have the troops on hand if need be.
I don't know how long it takes for a shit to get to Mars- Doom 3, between anyone sending the transmission to the end segment, it's implied to be fairly quick. Even so, it's not as reliable as having troops on site. And the big personnel transfer? They send you. One guy. Even Swann has only one man with him for backup. Either there's a serious shortage of Marine personnel for some unspecified reason(maybe Earth is in really bad shape with lots of armed conflicts?), or the UAC is really that stupid and has grown soft from power. I'm almost not surprised Hell was able to take the base apart, Betruger knew how lax things were. But then we go into ROE, where they know what's waiting, and they've still only sent a small number of soldiers.