I grow squash of a few types, though not what Americans mean when they say "pumpkin" generally. It seems to me that when I simply toss entire squash to compost in place,
volunteer germination is pretty low, as in
almost not noticeable among the chaos of my
polyculture. This leads me to suspect that the seeds are probably rotting along with the fruit.
Two thoughts relevant to your question follow from that observation:
1) This pattern seems like it could be beneficial for evolution: It would leave only the toughest seeds to survive and sprout. This could work against excessive density of seedlings. After all, as David points out, if every seed sprouted, that'd be too dense. The seedlings could still compete, of
course, but maybe a lower initial density has its pluses.
2) If you want this trait (namely, can be planted from slices or chunks) in your squash, why not pursue it? I mean, plant a few slices or whatever. You may well have poor germination the first round. But you don't need or even want every seed to germinate, if you're planting a slice. Then plant slices from those that do grow. You'll get better results. Repeat. And so on.
My thought on the other side, however, would be that part of what I select my squash for is a certain shape and flavor. I want to eat those puppies, and then save the seed from those exact squash fruits! So if a squash appeals to me visually, before eating, I scoop out the seeds and save its seeds provisionally. Then as long as the taste is good, too, I dry the seed and plant it later. I will say that I'm not very particular about cleaning the seed I save as a rule and I still generally get good germination.