Why mostly people are not serious about their career?

Because not everyone has a career; they just have a job.  There IS a difference.  Most people who have careers have advanced educations and degrees to prepare them for the field they are interested in or they've gone to a trade school.  Those people are serious about their careers and they should be - they've spent a lot of time and money in earning it.  I don't agree with your statement.

To me a job is something which requires little or no skill, doesn't pay well and there is little opportunity for advancement so there is no incentive to stay in it.  So why stay in a job if a person can so easily get another one at approximately the same rate of pay anywhere else.

Some get serious when they are laid off.

Because animals don't have careers. Therefore it's mostly people that aren't serious about their career.

Why mostly dogs are brown?

Why mostly men laugh at trees?