Because it helps the water evaporate ... compensates for the lack of wind and full sun.
With plain soil, or a mixture of it, you will be fooled (as I was, especially in the winter) by the surface soil that will dry out (at witch time you will hit it with more water) but the inner soil will always stay wet.
Implicitly your tree will get the roots to rot pretty fast.
It gets less wind and less photosynthesis due to the lack of full sun.
If you put it in dirt ---> the tree will get More water when it already has a smaller capacity to process it.
Not to mention the ratio of oxygen <--> water will be pretty unbalanced.

Claidiu... Maybe this link would help changing your approximation towards soil mixes. Hope it does

adamaskwhy.com/2018/01/30/akadama-the-ideal-bonsai-soil/

Alain

Thanks for the link.
I don't deny that trees can grow in other soil. Hey what works for you works.
I just say from the little experience that I have on the subject, that works best and it's easy to get.
You can grow them in any type of soil but when you see top trees I doubt you will see them in anything else.
Take a look at someone who tells the real deal:

I was talking only on topic of "indoor" trees btw.