Um, because the separation would also create a physical separation between BC and the ROC. People traveling to or from BC would need to bring extra ID like passports just to travel through AB. Flights originating in BC or the ROC would have to advise the US govt of every passenger that boarded a flight if the flight path took it over AB, which would be a US state in your scenario.
In the case of Quebec separating, do you honestly think they were going to leave with their current borders intact? Do you really think the Fed would have permitted Quebec to cut off The Maritimes from the ROC? So why would you think they'd let it happen with BC?
In 1998 the CSC ruled on the idea of secession. Here's the short version. A province can secede from Canada if and only if its residents vote to do so AND the other provinces and territories agree to the secession. A province cannot unilaterally separate on its own.
They will just have to extend the Trans Canada to the NWT
As for cutting off the Maritimes I always go through the USA and re-enter at Bangor
The Supreme Court of Canada ruled that a province may secede from Federation union. The Canadian constitution allows for it.
https://www.cidob.org/en/content/download/57092/1472197/version/3/file/171_OPINIO_EUROPA_ANG.pdf
The Clarity Act spells out the process for a province to secede from Canada.
A referendum with a clear question. The ballot question must only refer to secession, and the Canadian Parliament must agree before the vote is held that it is a clear question.
A clear majority must vote for secession. This is not defined, but the Supreme Court has ruled that it must be more than just 50%+1. The vote itself must represent a clear majority of the province, and be sufficient enough to maintain popular support during the likely difficult negotiations that would follow; if only 51% of voters voted to secede, there is a good chance that during negotiations and discussions of new borders for the former province, assuming the provincial share of federal debt, etc., the number of supporters could easily drop below 50%. The Canadian government gets to determine what is a clear majority.
Following a referendum in which a clear majority of a province voted to secede, the Federal government, ALL of the provinces, and the First Nations would enter into negotiations. Any secession would necessarily involve amending the Constitution, which would require approval of at least 7 provinces totalling 50% or more of the national population, as well as approval by the Federal Parliament and Senate. Provincial borders would need to be redrawn to account for First Nations land remaining in Canada; if the country is divisible so is the province in question. As mentioned earlier, the negotiations would also include issues such as the national debt, use of the Canadian dollar and Bank of Canada, use of Canadian passports, among other issues. All parties involved would be required to negotiate in good faith.
After the clear majority vote in a referendum with a clear question referring only to secession, after the negotiations, and after the votes to amend the Constitution by the Parliament, Senate, and seven provinces representing at least 50% of the national population, and with the approval of the relevant First Nations, a province will have successfully seceded from Canada. Negotiations can now begin with the US. I will leave that process to an American to explain; I will just point out that while Alberta's oil might make it an attractive addition for the US, they already have territories like Puerto Rico that have been in line for statehood for quite a while. Adding a state would be at least as complicated as seceding.