This. Because of the massive amount of hiring the big 4 do their entire interview process is available online. Sure there are variations depending on the interviewer you may end up in a room with but for the most part they all follow the same format. Anyone who really wanted to work at one of these has to study and practice how to interview for THAT company. Sure there is still some element of luck / right time right place in play but most people who fair interviews at the big four just don't put in the time required to prepare.
Intelligence? That's a fairly hard thing to measure and likely a poor indicator of a good employee. However there is a element of prep and memorization which does require a base level of intelligence.
Haven't gotta an offer from the Big4 but like I stated half of my former team now work at one of the big four and I've worked with loads of people who have received offers or currently work at the Big4. I also know countless people who have been utterly destroyed in technical interviews with the Big4 because they didn't put in the time and effort to study for it and treated it like a tech interview for a digital consulting agency.
