It's probably the same in canada and the UK, but academic advancement in the US has just as much to do with personal relationship as with merit. How much your professors like you and who they know is essential for how you progress within academia. Beyond recommendation letters, it's your professor's access to the academic network ( lab openings, research opportunities, etc ) and other jealously guarded opportunities. These professors are truly gatekeepers to your success.

"After graduation, Zhang had a hard time finding an academic position. In a 2013 interview with Nautilus magazine, Zhang said he did not get a job after graduation. "During that period it was difficult to find a job in academics. That was a job market problem. Also, my advisor did not write me letters of recommendation." The reason behind this is that Zhang's research pointed out the mistakes made by his advisor Tzuong-Tsieng Moh's previous work. Moh was very unhappy with this and refused to write the job recommendation letter for Zhang.[11] Zhang made this claim again in George Csicsery’s documentary film Counting From Infinity while discussing his difficulties at Purdue and in the years that followed."

https://en.wikipedia.org/wiki/Yitang_Zhang

What? The goal of a PhD is to train somebody to be a researcher and collaborator. The best way to do this is to collaborate with them on research. Collaborators deserve authorship.

I understand that many labs are hierarchical with ideas solely handed down from top to bottom, but removing advisor authorship is a big step.