Because it changes the right of way and so you have to stop start more than you would otherwise.
This has been done in places in Cambridge UK (a reasonably bicycle friendly city). If your cycle lane is part of the road you can act like a car does - you get to ignore turnings because anyone joining the road has to give way as per normal. If you're on the pavement though, you're like a pedestrian - every turn off is a road you have to cross. Even if there isn't any traffic you have to slow down enough to look and as a cyclist what you want is constant speed.
If you can't quite see what I'm talking about aee this example from Milton Road: http://www.rtaylor.co.uk/images/milton-road-1.jpg The 'main' road is the one going into and out of the picture with cars on, and the image pictures a junction. You can see where the cycle lane 'crosses' the joining road and how you'd have to look for traffic (even ignoring the bump down and up that messes with a nice cadence).
If you're talking about sidewalks with separated pedestrian and cyclist traffic, some bike lanes are effectively that (e.g. raised to sidewalk level, left turn boxes on the right instead of forcing bikes to cut across traffic). They don't seem to be very common. Perhaps it's because it is a newer idea that people are unfamiliar with or maybe it is because it is less expensive to take some space from oversized lanes than it is to widen sidewalks.
You mean put the bicycles on the same sidewalks as the pedestrians? Maybe it sounds like a good idea to you but to someone who lives in the Netherlands that sounds completely insane.
Unless you meant something like [1] or [2] where there is a raised bicycle lane effectively on the sidewalk.
But even if they're part of the road that's not necessarily a problem [3], but in that case you'll want to have a reasonable speed limit (that sign on the road is 30 km/h, not 30mph) and you'll have to teach cars they're supposed to just be patient and slow down if they can't overtake.
You can't teach drivers such a thing, they still will be impatient. That's why cars and cyclers can't peacefully coexist on the same lane, another solution is needed.
It works well when there aren't too many pedestrians and they don't confuse the bike lane for a sidewalk too much.
On the other hand, if there are enough pedestrians, they will spill on the bike lane thinking it's a sidewalk. Or in some places, people really don't pay any attention and don't have a single care in the world, and will be totally oblivious to the fact that they are on a bike lane, even after ringing a very loud bike bell a few times right behind them.
My commute happens to have both of those configurations, and the second one is hell, to the point I'll rather ride on the bus lane (which is forbidden to cyclists) than risk a collision with a pedestrian.
Edit: I almost forgot, but since it tends to be between the road and the sidewalk, cars turning right will always check for pedestrians, but often fail to give way to cyclists, depending on how used they are to see cyclists coming. Some intersections have high change of T-bone because of that.
Pardon if I'm misunderstanding the layout you're describing, but I do wonder:
1. How often do pedestrians wander into those bike lane?
2. What are the rules at intersections? Specifically, does a bike lane along a major road have priority over minor roads at intersections, or are bikes required to give way to all traffic from both the major and the minor roads?
3. How obvious is it to drivers turning right from the rightmost lane that they might, in fact, be crossing a bike lane?
1. Depends on where. In busy areas it happens a lot. You just have to be careful when there are people around.
2. The bike lanes have their own traffic lights, which are on the same cycle as the cross walks to the right of them, and the main road/lights to the left.
3. It's obvious since these bike lanes are everywhere; it's just apart of driving. I will say it's a little worse in the West of the city though!
All the answers to this questions have a simple guideline for cyclists, drivers and pedestrians altogether: slow down if you can't predict what will happen next. There is no magic behind it. Someone won't pay attention and the difference between a crash or a near miss is usually at least one paid attention and slowed down.
1. Pedestrians do wander into bike lines quite a bit but I think it's improving because of the sheer number of e-bikes and scooters out there now.
2. The sidewalks and bike paths run together, and road crossings are dual marked. Everyone needs to exercise extra caution, of course.
3. Riders for the most parts have the same traffic flow as pedestrians. If you're turning right, you'll most likely cross both the sidewalk and the bike path at the same time.
Definitly a good question, given that pedal bikes exist in a grey area of traffic definitions in many areas, but I will say in my area (small urban area of 150k population) 95% of bike lanes are in areas without sidewalks, just dirt shoulders.
