The simple answer is because they don’t know how it works.
Psychics, Omens, Fortunes and Prophecy are ancient gimmicks that are almost as old as prostitution. One thing they all have in common is a message or warning that has no date of arrival - keeping its believers guessing into eternity - and making it a prediction thats good until the end of time. When given enough time, an event with similar themes will eventually arise and the warning is then verbally massaged to make it appear true. Some prophecies have been perpetuated for so long, that dozens of events have been repeatedly attributed to them.
The psychics and mediums of today like the one your friends went to are just modern versions of ancient prophets. Their business models are different but they use the same trick. Recently, they have come up with even better tricks that actually use science to deceive people - the art of Cold Readings. Its based on psychological types can be classified into 16 different categories with 4 groups of 4 types:
1---------2---------3--------4
ENTp…ISFp…ESFj…INTj
ENFj…ISTj…ESTp…INFp
ESFp…INTp…ENTj…ISFj
ESTj…INFj…ENFp…ISTp
E - Extrovert
I - Introvert
S - Sensing
N - Intuitive
T - Thinking
F - Feeling
j - Judging
p - Perceiving
Psychics combine these 16 type classifications into only four and use a system of general questions designed to probe the compressed groups quickly - mainly by body language and voice characteristics. The probe begins with a very general target of the entire audience such as “I’m seeing a name that starts with L” or “I see a significant date with the number 14”. The probes drill down the audience until they find a person who is gullible, vulnerable, or susceptible to suggestion - basically people who fall into the Introvert groups.