Personally, I see several reasons for it.
As a rule, the first is submission. You are expected to obey unquestioningly in order to reach a level of initiation.
The second is insecurity. The “guru” is so dependent on their system that they are afraid to take a step to the left or to the right.
The third is plain stupidity. One “guru” repeats what other “gurus” said, and they repeated what their own predecessors said… But why? Tradition!
I’ll tell you about my own path: a complete rejection of authority! Even Crowley is no authority to me if he gets in the way of my path. But I use him as a springboard for building my own space of magic.
There should be no authority over you. You must achieve things for yourself.
Why do I think this? Every person perceives the world differently, and our spiritual worlds are so different in content that there can be no single map, no single path, and no single set of correct actions!
An approximate plan is all we have. Spirits and archetypes are like markers on that plan. And on their own, they have no meaning. Meaning comes from our own being: it is what we use to fill both the archetype and the entire space around it.
It is Meaning, not Form, that matters.
It makes no difference whether you use the image of Jesus or the Devil. If you fill them with the same meaning, that will be right for your personal myth!
So traditions… They are a good starting point for studying magic, but not for practising it.
Abramelin the Magus wrote that it is not the magician who exists for the stars, but the stars that exist for the magician. The point is not to cling to form, but to work with content.
I am critical of gurus. Teaching magic is impossible without building a personal space, and that is work each person must do for themselves. All you can give someone is a plan with a few markers. From there, everyone goes their own way.
I rejected every “rule” of magic, and what happened? I’m alive, and mentally healthier than 90% of the ‘schizoterics’. I haven’t drunk myself into ruin or become a drug addict. I look at things soberly and assess the worlds of magic and spirituality realistically. I recommend the same to everyone.
Be critical. And I strongly recommend studying a materialist model of the psyche alongside your other studies. It really helps keep you from losing your mind over idealism.