I'm not sure why you think it does not happen most times?
How would you know? Most people don't speak about these events openly.
I can only judge by what I've seen. Our ego lives a lot of our lives and doesn't want to be seen for what it is. it means its death. It shouldn't be surprising how difficult freedom actually is.
Often a person does experience the Spirit to a certain degree but it is quickly adopted by our ego and the result is often worse than before. It is explained in Matrthew:
Matthew 12: 43-45
43"When an evil[a] spirit comes out of a man, it goes through arid places seeking rest and does not find it. 44Then it says, 'I will return to the house I left.' When it arrives, it finds the house unoccupied, swept clean and put in order. 45Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there. And the final condition of that man is worse than the first. That is how it will be with this wicked generation."
The cleansing effect of the spirit leaves a person inwardly vulnerable. If not complete, the same tendencies return and even more powerful. I've seen this to be true and most often with superficial proselytizing
