Why don't commercial aircraft adopt a slightly more seaplane-like design to allow safer ditching in case of emergency?
Because ditching is extremely, extremely rare, so the costs of redesigning aircraft along with the extra drag and weight (increased fuel burn) it would no doubt add to the airframe far outweighs the potential benefits.

That argument might sound weird to someone, but think about it this way: would it seem reasonable to redesign every single car on the planet so that they can float, just because every year a few people around the world accidentally drive their car into a lake?

A quick search on The Aviation Herald suggests that, since 2005, a total of four flights worldwide have been forced to land on water. To give you an idea of the scale, there are about 100,000 commercial flights worldwide daily. So that's something like 4 flights out of half a billion, which is practically nothing. Also, in those 4 accidents, more than half of the people onboard actually survived. Actually, the increased greenhouse gas emissions caused by adding extra weight, such as floats, to aircraft worldwide would probably kill many more people than such safety equipment would ever save.

It's not that the aviation industry isn't investing heavily to improve safety. It's just that making ditching safer is an area where there is almost nothing to be gained. So the money can be much better spent elsewhere.