Quite a lot of cyclists use the sidewalk because they think it's the correct place to cycle and/or safer. This leads, predictably and empirically, to the typical behavior of sidewalk cyclists.

Since they typically move faster than any pedestrians present, they will be inclined to pass them.

Pedestrians often walk in the center but, because they see other pedestrians coming the other way, it's simple for them to move to the right. They don't see or hear overtaking cyclists which means cyclists will be inclined to weave around them rather than stop/slow to wait for them to move over.

Cyclists (in the US) don't generally use bells and the few that do tend to use them to late/close so that the pedestrians are surprised and move in unpredictable ways.

And sidewalks are typically narrow, making all this stuff worse.

Given all the complications and limitations of using the sidewalk, cyclists using them tend not to be riding very fast.

Most of these cyclists are riding to get somewhere. It's a small part if their normal lives, which means they are not inclined to do anything special to prepare for it, which means they are going to be in their normal clothes (dark clothing is common and normal) and not have/use special equipment like lights. And, as far as they can tell, they see just fine without lights (and might guess that others can see them just fine too).

The fact that they don't usually have any problems reinforces the behavior.

"Real" cyclists have so much experience riding in a very different way that they think their way of riding is the "normal" way.