The answer is really simple ... Routine + the fact the brain is really good to see what he want to see ...
How many accident / incident are due because people after doing the same check and SOP for few years, finally got a day : they think they did it, they looked at what to check, they did the call accordign to SOP but in fact they didn't see the selection / switch position ... was wrong.
You really need to force you to do the check and even we are all Human.
I remember in the SIM few years ago, after a go around, I did the check list, I had a look to the no light of the landing geea and happily called "gear up, light out", during the radar vextov I thought the plane was noisy but it didn't trigger anything in my mind, when PF called for gear down during the funal, I realized the gear was already down with 3 green, I then apologize, the captain and the trainer realised at that time the gear were down all the approach, 3 people in a sim ...
Now I agree with many of you, was a lot of possibility to catch the problems sooner ... When they did the taxi check list, was a check of the AP mode, when they lined up, was again a check of the AP mode engaged, I m sure they did the right call and I m sure when investigator asked the crew, they both remember all was fine and set as they called with GA mode engaged (plane was in alt mode on the ground)