Perhaps in some cases because gears are knotty-plexicated.
As a child I had a bike with 3 gears. It was never explained to me what they did, so I just pootled around on my bike as kids do (probably in 1 all the time).
Then I learned to drive and got the idea of what gears do.
Then I got a 21-gear bike. And I could not work it out. Because nobody explained to me that on a bike, you don't start in 1st and make your way up to 21st. I could get moving in 1st (i.e. 1 on both gears), and quickly find myself moving up the gears until I was in 1/7; but then what? My next move would be to 2/7, when surely I'd want 2/1????
Yes, you are all laughing at this. But it pretty much kept me off the bike for 10 years - I bought it, I couldn't get it to work, it sat in our hall gathering dust. Perhaps I should have gone back to the very nice LBS who sold me the bike, and had it explained, but it seemed too stupid to ask. My husband - not a cyclist - found it very amusing, and said 'but you drive a car,you know what gears do'. Basically, I thought I should understand it, and the fact that I didn't wasn't something to advertise!
It was only when someone very patient explained to me what the 'left thingy' and the 'right thingy' were actually for that I understood. And started to use my bike, with its 21 (but not actually 21) gears. And now it seems straightforward, and I could explain it to someone else. But equally, my mother is still better off with her simple 1-2-3 hub gears!
I think that the bike you buy from a not-a-bike-shop these days is a multi-geared hugely suspensioned mountain bike, not suited to what most people actually need. And if they don't get the nuances of how it works, well, they can just pootle along with how it is set up and so that's what they do.
for bso riders.
By 14 it was rideable trial bikes (with motor) and by 16, driveable (by some one else, anyway) mini mokes 
So a Sturmey Archer was never going to hold any mystery.
