The reasons for 1 and 2 are the same: refineries are huge, expensive, and difficult to build, taking a long time and often a lot of wrangling. Case in point, even in the rather oil-friendly environment in the US, there hasn't been a single refinery built within at least the last 20 years, maybe more. Houston is where the US refineries are, refineries which already have enough excess capacity to handle oil being imported, refined, then returned elsewhere. This is doubly true when you already have people in Canada raising hell about the ecological impact of tapping the tar sands in the first place, let alone then building a refinery there to further crack the hydrocarbons.
3. Possibly, but the fact is that the "It's all going overseas!" mantra is greatly overstated. Selling oil on a world market, it's not really that cost effective to take oil out of the US and ship it halfway around the world where they're going to be paying basically the same price. The net result is that some would end up going overseas, some would be used in the US.
4. Building a pipeline isn't really a lot of construction jobs. It's basically just a massive underground pipe. You get a short burst of construction work, then very little afterward. The real windfall to Canada is number 5, for which the answer is "Because they want to sell the tar sands oil and make money off of it." That's where the cash is from the Canadian perspective, and it's why now they're talking about trekking it to BC and shipping it to China (as I pointed out at the time of the Keystone discussions).