They expected to get it designed and built within a few years with no interference, not even for the International crossing. Similar transcontinental pipelines from Canada and into the US had never faced such opposition (much less, common knowledge of their existence.)
Second, a refinery takes at least two years to design and build from scratch. Cost is about a billion, or more, depending on its capacity. I don't know what the capacity of a single plant is, but would guess most can't handle the projected 800,000 barrels per day without splitting it up among other plants.
So, if TransCanada had to built their own plant with the capacity to handle that much bitumen per day, it would likely have cost them another couple of billion. While they would have finished either the pipeline or a new plant at about the same time, refining it at their site would still leave them with the problem of shipping the refined product to market. I don't know what the shipping capacity is for their ports, but would also guess they can't handle the surpertankers that would likely be required to get that much product out per day, or even more 'regular' sized tankers for the equivalent carrying capacity.
None of that is a problem for the Ship Channel in Houston, where most of the petro-chemical plants here are located. The port in Beaumont/Port Arthur also has the capacity for such amounts. I thought I'd seen a map showing a branch of the pipeline heading that way for processing, to seemingly further spread out refining capacity.
I hope that answers your question. Oh, Oklahoma is known as a primary distribution center for many of the major pipelines going across the country. Their refining capabilities are limited compared to what's on the Gulf Coast.
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