What prevents companies like Cessna and Piper from connecting multiple alternators to a single engine?
You almost answered the question yourself.

Having more than one alternator would introduce more complexity not only mechanically but also in the electric system (even if they work correctly). Alternators scale really good so that a bigger alternator generates more electric power then two smaller of the same weight.

So the only reason to do that on a smaller air plane is to have a backup if one of them fails.

This brings up two questions:


How often do alternators fail?
what are the consequences of a failed alternator?


For the first question I have a (non representative) experience:
At my airfield 6 SEP plane are are posted. In 25 years I'm flying there we had 1 failed alternator.

For the second question we have to look in the flight instructions and the planes manuals. For smaller planes like those you mentioned they state that you should turn off some electric devices and land on the next reachable airfield. That means there is no immediate risk when an alternator fails.

The conclusion is that we simply don't need more than one alternator in a SEP plane.