IME it's more that the advisor doesn't trust the student to make it through the annual layoffs (quals culling), and only wants to invest in people who they know

IME it's more that the advisor doesn't trust the student to make it through the annual layoffs (quals culling), and only wants to invest in people who they know will be around long-term. At least in the poor (and honestly mostly useless) parts of Mathematics. Maybe Physics is less poor. (Fortunately I was in CS, where the research output is actually needed by society and usually not pure masturbation, so the attitude toward coursework was "do well at what you need, enjoy what you want, and ignore what you don't need or want"