It's a trend, that's all. Give it a few more years and all that will remain are a bunch of vandalised classic frames that we will look at sadly, wishing those lugged and brazed steel beauties had not been modified for some hipster's vanity.
There's one actual reason which is lower maintenance - less complexity does mean less to adjust. Get your chain tension right, keep it lubricated, things should stay smooth for a long time.
Against the fixie there are so many more - being in the right gear only 10% of the time is the big one. A fixie is for the track where the gradient is zero and the road is always smooth. (You go up the embankments of course but race tactics is a whole separate, big discussion.)
"trackstands" - many skilled cyclists and even a lot of plain old commuters can balance indefinitely on mountain bikes, cyclocross, road bikes or whatever.
"direct connection" - huh? There's no slack in my drivetrain, nor on any properly adjusted bike. There is the direct link that keeps your pedals rotating any time the wheels are rotating of course, which can be very dangerous if you lean into a corner and find your inside pedal lifting the back wheel off the ground as it comes around...
"lighter" - modern frames and group sets are so light anyway. The UCI (governing body for the world road championships etc.) has set a minimum weight of 6.8kg because it is easily possible to make a lighter geared bike than this. There's a point beyond which it just doesn't matter any more, and both geared and fixies are there already.
"efficiency/fitness" - there are two sides to this. A fixie can help you learn to keep constant pressure on the pedals I guess. But gears are there for a reason. Lance Armstrong doesn't have the cadence of a hummingbird because it's more fun that way, it is more efficient. When you're travelling downhill fast and your legs are spinning to keep up with your pedals that's awesome, but it's the time you least need efficiency. You need it going uphill, but that's when you're standing up in the saddle pumping slowly. Which leads to a warning - fixies may be bad for your knees. If you ride in hills and you tough it out with a tall gear you will cause more wear to your knee and hip joints than if you were able to change down and keep your legs spinning. 20-somethings, you think you're invincible now but you may well regret that attitude once you turn 40.
Brakes don't enter into the debate because a fixie can have them or not.
Take your fixie shopping on weekends. For riding with an actual purpose, accept that technology has advanced a bit. As per Lance Armstrong's cadence, they don't ride bikes with gears in the Tour just because it's more fun or they like to tinker.