Well, you may not be able to “see” it, but that’s how it works.
In order for a bishop to approve an apparition, he would need to conduct an investigation. Some dioceses simply don’t want to bother with this; it costs money and resources, and the bishop may have other priorities, like simply providing the sacraments to his flock with a limited number of priests, or balancing the budget, or something else. An investigation also might be difficult or impractical, for example if the alleged seer passes away before the bishops bother with investigating. Even when an investigation is conducted, there might not be enough conclusive evidence to say something supernatural definitely occurred.
But if the seer’s messages are encouraging people to pray and become closer to Jesus, then the bishop might think it’s good to promote the devotion, even if he doesn’t feel he can investigate or say for sure that Mary definitely appeared under the pine tree in the park on date X.
There are other cases where the bishop doesn’t want to approve the apparition because the seer has a questionable character or claimed a whole bunch of additional apparitions where the first ones look perhaps authentic and the later ones not so much. For example, Our Lady of Revelation had a seer who had a very questionable past and had left the Catholic Church for the Seventh Day Adventists. He claimed that Our Lady appeared to him when he was on the verge of killing the Pope and that she stopped him from doing this. While there are a lot of reasons to think Our Lady actually did appear (including the fact that she appeared in the same spot earlier to another totally different unrelated visionary and told her to convey the message to the Pope that someone was going to try to kill him, etc.) the bishops apparently don’t want to officially approve this apparition because of the character of the seer. But they allowed a big shrine to be built there, and Masses said there every day, and many people come and pray and it bears good fruit.
I also understand that in cases of interior locutions rather than an apparition, then no investigation will be done. In that case, the only possible approval is to have the messages of the locutions “approved for faith expression”. I reckon the Church figures there is no way to prove someone did or did not receive an interior locution, since it would be just Jesus or Mary sending a message to the person internally, rather than appearing in a visible way.