If you can play video games and memorize profiles, you can pass sim training. The problem is, you hardly EVER fly the profiles in real life. That leaves the new FOs totally clueless as to when to slow down, drop flaps and drop the gear. IOE can fix SOME of the problems, but you don't have near enough time to fix them all. Plus, you've got scheduling working against you. For example, I was having trouble with my visual approaches, and all of my OE CAs knew it. They told scheduling that. So what does scheduling do? Gives me a 4 day trip with 2 DHs, long overnights and about 6 landings. I wound up having to have my OE extended to get more visual approaches in.
Plus, no 121 training in the world can substitute for raw experience. As a OE CA, they can't say "Well, you've never been in IMC, so we're going to go find some" like a CFI can. You may or may not get a diversion on OE. My first diversion was two weeks after OE. You may not even get any significant WX on OE. As an FO, you can get very lucky, skip all the major decision making events and breeze through OE. That first CA I was talking about (the one with the guy with no IMC) told me his FO even said a few times "I don't know what's wrong. I flew the sim fine. I don't know why I'm having trouble with the real thing." Truth is, the sim only flies somewhat like the real plane. I went back into the sim for Cat II training, and it was UGLY. I was in the habit of the control forces (or at least the hydraulic equivalent) of the real thing and wound up landing when I was trying to go around b/c I didn't pull up hard enough. If I had done it like that in the real plane, I would have at LEAST gotten the continuous ignition, maybe the shaker.
The point is, no 121 training program can identify all the gaps that could potentially exist in a low time pilot. They were designed to take pilots with a moderate or better amount of experience and transfer that experience to a bigger, faster airplane. The programs were never designed to take someone with little or no experience and get them that experience before they're ready to go out on their own. To truly make the programs work for low timers, IMO,they'd need to be totally re-designed. In our situation, we're too understaffed and too short on check airmen for us to extended OE 40 hours because an FO doesn't "get it."