I would question that assumption. I would say the evil ones prefer whatever is most advantageous to leading people astray. Sometimes that is remaining hidden. Sometimes not. Although, appearing as a false vision would still be “hiding”, no? It’s not as though they are appearing in their true form as vicious demons.
As to why God allows them to engage in such charades, that’s a tougher question to answer. Here’s what the Catechism says:
CCC 395 The power of Satan is, nonetheless, not infinite. He is only a creature, powerful from the fact that he is pure spirit, but still a creature. He cannot prevent the building up of God’s reign. Although Satan may act in the world out of hatred for God and his kingdom in Christ Jesus, and although his action may cause grave injuries - of a spiritual nature and, indirectly, even of a physical nature- to each man and to society, the action is permitted by divine providence which with strength and gentleness guides human and cosmic history. It is a great mystery that providence should permit diabolical activity, but “we know that in everything God works for good with those who love him.” [Rom 8:28]
By calling it a “great mystery”, they kind of dodge the question.

We don’t really know for certain. I would venture to say that a lot of it has to do with free will (even the free will of the fallen angels). God doesn’t squish those who do not choose Him instantaneously. That would effectively eliminate any opposition and somewhat undermine free will. If the choice is between God and being immediately snuffed out of existence, our choice for Him would be one borne out of fear rather than love. And that wouldn’t be a free choice.