Inverse square law.

Supposing you are trying to tap the energies from planet X to cast the universal problem solving spell, fireball.

Supposing planet X is at its farthest, you can cast a fireball the size of a baseball. Half a year later the planet is at its closest and now your fireballs are the size of basketballs.

Everytime you halve the distance to a planet, the amount of energy you get from it quadruples for the same fissure size.

So if by tapping a planet that is a few light minutes away you get enough energy to create a basketball sized fireball, how fast will thermodynamics turn you into plasma if you try to tap a planet that is zero inches away from you?

The [local mage guild](https://worldbuilding.stackexchange.com/q/116262/21222) has probably gone quite out of their own way to prevent obnoxious mages from becoming walking nukes.