Because in order to have a true vision from God one must be completely freed from passions, cleanse his mind and heart and confess faith rightfully.
“In the fallen state, out of all feelings only one “can be utilized in invisible worship: sorrow for sins, for sinfulness, contrition of spirit . . . Sacrifice to God is a contrite spirit: a contrite and humbled heart God will not despise (Ps. 50:17)” (Bp. Ignaty).” - St. Ignatie Brianichinov
A mystical experience can come from devil and trick you into wanting more of that. And in the beginning the feelings it gives will be pleasant, use Christian iconography, our expectations, fears, you name it, until you give in to it and ask for more. Once the devil has you he can pretty much do what he wants with you. People have committed suicide thinking they can fly and others simply lost their faith. After the devil catches you he can quickly show you a merciless God for example and you lose all faith right then and there and walk away happy with a false feeling of liberation.
The best mystical vision is humbleness and patience. Seeing one’s own sins without the self-judging but asking God for help. That keeps you safe and it is a real mystucal vision not to be despised at all.