By coincidence I was having a conversation with someone who earns a living in part making midi files for retail organisations. He mentioned in passing how he hoped to finish the job soon to be within budget. This is something that is easy for us to overlook.
At home I can spend as long as I like practising and perfecting the patterns for a song because I do it for my own pleasure and personal sense of achievement. Learning the melody and chords is relatively fast compared to the time I spend perfecting the pattern backings so it sounds just right. Presumably these guys work in a similar way, only without so much pattern backing, maybe listening to the audio track while trying to figure out how the backing needs to be played.
If these guys are paid fixed contracts they can't spend days on each song because they have to get on to the next job and earn some more money. That makes the skill involved making these superb files all the more remarkable in my book.