Building more refineries in Canada would then necessitate them building an extensive pipeline distribution network for finished product to get it to market...in the USA.
Helluva lot more efficient to build one crude pipeline to the Texas Gulf Coast and let them distribute the finished product to market thru an existing pipeline network.
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That may well be part of the issue...but it’s also the case that the oil coming out of Canada is the heavy crude oil and there are few refineries that can accept that type of oil. It’s also the case that the Canadian government is having issues w/ Canadian citizens pushing back about building more pipelines and refineries in Canada.
I’ve also ready that as the US continues to tap our own resources for oil...our product is the lighter ‘sweet crude’ and we might, in fact, use up current US refinery capacity and have no need/reason to draw on Canada’s crude oil.
A number of refineries on the Texas Gulf Coast are equipped to handle this kind of oil -- it's similar to what's being imported from Venezuela, e.g.
Wouldn't you rather be paying for and refining Canadian oil than Venezuelan oil?