Table field type defaults
I just did some testing of this. I thought I noticed the same thing, but then realised that I created and added the field to my section (a single) only after it already existed. In that case the defaults didn't show. But then I added the field to a channel section and created a new entry and the defaults show in that case.

So I think the reasoning is that it won't populate defaults on an already existing entry as these could easily go unnoticed by someone editing and end up being saved to the entry when unintended.