Why do bikers ride on the street instead of the sidewalk?

Because most states have deemed bikes to be vehicles and they belong on the road and made it law! There’s your sign!
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In most municipalities it is illegal for anyone above a certain age to ride a bicycle on a sidewalk. Most consider the bicycle a vehicle, and therefore entitled/required to use the roadway.

The sidewalk is generally more dangerous to ride on anyway because of pedestrians, and vehicles crossing the sidewalk at alleys, driveways, etc…
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Because depending on where you live, they are probably not allowed to. Sidewalks are for people walking, especially families and children. Bikes travel at a much faster speed and there is a possibility they could collide with pedestrians.

I agree that it seems somewhat risky for them to be riding on the street with traffic, but that is generally how it is all over. The two have to learn to share the road.
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Putting aside the legal issues, riding on the street is safer and faster than riding on the sidewalk. On a sidewalk, anyone can walk out in front of you without warning. There can be obstructions such as trash cans, etc. The expansion joints make for a rough ride. Low hanging trees are often a head banging problem. There may not be smooth ramps on and off the sidewalk when you reach the end. A drop off a curb can give you a pinch flat on a road bike. And the most important — people don’t expect to see a bicycle ridden on the sidewalk. They don’t know how to react, making it more dangerous for both the pedestrian and the rider.

HTH
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In Illinois, it’s ILLEGAL. It’s funny ’cause I ride…alot, and there are times I run into drivers that say "Hey, you should be on the sidewalk!". I scream "Pi$$ off a$$hole, it’s illegal!" Hence my screen name. HA!
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You walk on a sideWALK. You ride on the ROAD.
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These are all great answers. Let’s hope people who post questions like this read the responses and get a clue about cyclists and their legal and safe rights to riding on the road.

M.
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