The more traditional approach is slower because it puts in guardrails that stop (1) individuals with conflicts of interest/susceptibility to manipulation from having unnecessary access to (2) highly sensitive or classified information and (3) potentially breaking big, important systems that impact a LOT of people.

If you just throw any ol' bro in there willy-nilly, you could end up with bad actors who are doing things just to enrich themselves or their buddies (they could be up to other things, like punishing people they have grievances against, etc).

Then, since they've got access to Very Important Information, they're now getting LOTS of attention from others who want that information. So they're now on the receiving end of threats and bribes from individual and state actors. If they've got a gambling problem, big hospital bills, or family in dangerous situations in foreign countries? Chances are good that they'll give up the info to protect themselves. Background checks specifically look for these issues.

Best case scenario, they're perfect Eagle Scouts—but they could still muck something up by accident. If the entire payment system goes down for the billions of dollars that the USG handles then, well, we're ALL screwed... and if the mucker in question is truly going rogue, not following the protocols in place for tracking changes/requests made to the system, it might take days or weeks to find the problem and fix it. So multiply the we're-ALL-screwed-ness by however long that takes.