Those bikes are small engine, low torque and consequently have smaller/narrower chains. They are designed for use commuting to/from work with little or no protective clothing over your normal clothes. Consequently the chain needs to stop spatter getting on your clothes and as a result has a chain guard.
The chain - due to its smaller weight and less torque of the engine will likely stretch less than a bigger bikes and should in theory need tensioned less often.
Larger bikes have much heavier gauge chains that have a lot more mass with them. The larger bikes also have much more torque applied through the chain. In the event of a chain failure the guard would have to be much thicker than on those bikes. Additionally the chains on larger bikes need to be adjusted more regularly (in my experience) and are less tolerant to slack/tightness so you need ready access to them.
That being said, larger bikes to tend to have a nominal chain guard to stop laces/clothing etc from the pillion passenger being taken up by the chain.
Also the chain covers dont look very nice compared to a gold x-ring chain and anodised sprockets.