I suspect that's because for the most part it just siphons off the traffic that would have mostly been a waste to serve anyway, being that they were either looking for such a small bit of information to be encapsulated in the excerpt, or they determined they were looking for something else, they would be hard to monetize. Then again, I could be wrong or there could be usefulness to users hitting the end site that isn't directly monetizable (brand awareness).

I wonder what percentage of the knowledge graph result excerpts are full entries, and don't trail off indicating more can be found on the source site.

Not saying you're wrong, but one strength of Wikipedia is the density of internal links, which even the summaries are full of.

Perhaps some users would explore a topic if they saw their many options to do so.

This is just pure speculation, though.

I think by now most people in the English speaking world know that if they want to start getting more depth on a topic then Wikipedia is a decent place to start. However, sometime you just want a quick summary/single fact.