Regardless of how you feel about it, the monarchy is totally intertwined with every aspect of Canadian law and governance. You can't just change the constitution, and that would be a herculean feat in itself. You'd have to change dozens of laws to remove any reference to the Crown, etc.
Finally, we still need a head of state. If it's no longer the GG acting on behalf of the Queen, it has to be someone else. Either we have a ceremonial president which is similar to our current GG (likely appointed so they never have democratic legitimacy), or a more powerful president like the Republic of France. Or we abandon our parliamentary system entirely and move to a presidential system similar to the USA. You can't just get rid of the head of state. The buck has to stop somewhere.