Why aren't the cyclists riding in the bike lane/road shoulder/far right edge?

I often see complaints by a subset of drivers who probably don't ride their bike on city streets that cyclists are often seen riding in the traffic lane when there is road shoulder/bike lane available. So, why don't cyclist always ride in the bike lane/road shoulder/far right edge of the road?

People don't always check their six before opening their traffic side car door...

You can't see this from the car, but 3/4 of this bit of bike lane is covered in broken bits of glass.

These won't do anything to car tires, but road bike tires go flat riding over them. Even the ultra-tough Gatorskins aren't immune to punctures!

Pine cones & other fruits are cute as long as they aren't in the bike's path! Also, when there are big trees lining the street, there tends to be root bumps and cracks right under them in the bike lane/road shoulder.

I added the vertical red bar to make the ickiness of this root bump more visible... Try hitting that thing while going at speed on a bicycle. It can really destroy the wheel (among other things)!

A bit of sand on the road won't cause a car to skid, but these can really cause a bike crash.

We cyclists haven't got a windshield.. Try riding into protruding tree branches at 15 mph. It hurts!

Branches are hard to see, even from a bicycle. This particular limb was at throat level...

You wouldn't drive a car over this, why should a cyclist ride his bike?

With all the broken glasses and radial tires' wire bits on the road, I get punctured most often from goathead thorns...

Some roadside protective ramps have ends that jut out. This one will easily take a shin off a cyclist and send him spilling into the traffic lane.

Every week from the night before to the night of the trash collection day, of course, there are extra obstacles in the bike path/road shoulder.

Also... a lot of time it is safer for a cyclist to ride left-center of the only through traffic lane when entering an intersection if he is behind a tall car so that the cars coming from the opposite side of the road that might be waiting to make a left turn would see him and know not to try to turn right as the tall car in front of the cyclist is going past.

A lot of street corners here have cars parked right up to the intersection. A street-savvy cyclist going straight will also ride more toward the middle of the right-most through lane rather than far to the right so that cars coming out of the side street will be able to see him (drivers tend to just look to the middle of the road for other cars before they make the right turn onto the cross street... and end up hitting a cyclist they should have seen but didn't).

So, the next time you see a cyclist riding in the middle of the traffic lane, please think about what legitimate circumstances that might force him to be riding there (believe me, most cyclists are sane and really dislike riding in traffic, surrounded by cars) rather than to immediately go into the road rage mode and assume all sort of evil motifs on the cyclist's part.