Why are zodiac signs taken seriously?

Exactly so.

What you call your "Sign" is ONLY your SUN sign, and just 2% of your entire natal chart.  Everyone has ALL 12 Zodiac Signs in their natal chart, but how much and how strongly we express each sign depends on the individual's natal chart that is unique to them alone.

And even then, ALL you see in the natal chart are psychological tendencies that will prevail IF the person does nothing to change or grow.  It doesn't predict others and does not control what happens in life.

WHY do people take astrology seriously? Because life is uncertain and human nature is complex and often unpredictable.  It is normal and natural to seek some understanding, some "tool" for trying to get a handle on life and the people we encounter. And the more unhappy WE are, the more we repress our pain, which also represses our self-awareness, so the more confused WE become about who WE are.

Sun sign astrolgoy - while NOT real astrology - offers a simplistic easy-to-find, easy-to-learn, ease-to-use "tool" to try to give hope to the desperate and unhappy, give understanding to the confused, give a false sense of security to the insecure, and give easy "answers" to those who are either mentally lazy or not very bright.

Writers make lots of money off the gullible, writing about Sun signs. But it isn't real astrology and it isn't even reliable.

I am the daughter of an astrologer, and have been studying and doing astrology myself since 1972.  Have the professional certification in astrology from the American Federation of Astrologers (P.M.A.F.A.).  Did consultations (no, we do not call them "readings") with clients for decades but didn't take those who expected too much from astrology or who wanted to be spoon-fed who they were.  Consults are two-way conversations between the astrologer and client, and take 1 to 2 hours .. so they are never written, and they help you discover yourself, rather than telling you "who you are". I have been retired for about 10 years now, and come here mainly to try to educate those who do not wish to be educated, but who are misusing Sun-sign astrology and harming themselves as a result of their lack of knowledge about the severe limitations of Sun-Sign astrology. 

There are many types of astrology and they don’t mesh with each other.

Your education didn’t include any fortune telling. Why would you want any part of astrology to be real. Show it works and how it works otherwise it doesn’t.

There is more to astrology than your sun and moon placements. Everyone has a chart at their time of birth, and the chart has 12 houses ruling different parts of your life. All your planets are in signs and fall into these houses. How the planets aspect each other has a lot to do with you and your personality. A prominent Mercury can make you talkative, as can a Gemini ascendant. (Rising sign) Go to alabe.com and fill out the birth info to get your chart. People that say astrology is bullshit usually know nothing about it except that they "don't fit their sign" or know someone of a certain sign that doesn't act as the sign describes. They seem to think the world is divided up into 12 types of people, and astrology is much more complicated than that.

Pride and Prejudice. They DO know that not all Aries or Geminis are the same because they were born on different days, have different planetary aspects and placements, different genetics or upbringings (religion, culture, environment). They just use the stereotypes of the Sun signs and the symbols (for example, lions for the Leo).

People take even age seriously, apparently if you are born in 1950, it means you know better, experienced and deserve respect.. people are ageist and likes to discriminate