Plus, at least in my region there always seems to be some sort of obstacle blocking sight in the center of larger roundabouts. Maybe it's just a convenient place for memorials, statues, etc., but I can't help but think that slowing down drivers is at least a welcome side effect.

The obstacles are intentional. Blocking sight forces people to slow down.

Roundabouts are the very last place where I would consider a bike lane to be a possible net win in safety. If the bike lane has priority at exits, then it will be constantly ignored, if it doesn't, then cyclists would be at the mercy of exciting drivers never forgetting to indicate, resulting in similar bloodshed. Besides being totally shitty towards cyclists who would then have to yield three times for going left at a crossroad-equivalent, at a very uncomfortable viewing angle and with permanent risk of un-indicated exits.

I've been curious as to why American roundabouts include unnecessary vegetation or a hill in the middle? Why obscure oncoming traffic?

If it's a roundabout, the traffic across the roundabout is not actually oncoming, it's the traffic approaching from your left around the roundabout that you need to watch out for.

The obstruction in the middle also makes the existence of a roundabout more obvious.

Here‘s an example of how they are handled in Germany:

https://goo.gl/maps/cmLVxuhViyF2

The space it requires is not much bigger than the traffic light. That town replaced many of its intersections with roundabouts even in central and older parts of the city.

(You might say they went a little overboard with it, looking at the southern limits near that hospital.)

Look at what they have in Berkeley. It addresses exactly this situation