Why most of the times people consider someone's 'goodness' as 'weakness'?

Good people : many of them are lenient- they know the real consequence of their reactions- both sides, and  have  the required insight over many issues by their experience and quality of thinking and acquaintance by learning by reading.

Ex. Suppose a subordinate is in drunken stage while at work- sometimes good manager may resort to a silence by thinking over the consequence of reporting mainly considering his family backgrounds-this guy may presume that he is a little bit feared of him- that is why he is not reported.....

because those two, with other will make you decide in a certain way, so knowing this of other people gives you information about there decisions and how they will act

It happens most of the time. That's why people say "Be good but not blind".

Being good for people who are worth it not all... As being too good is also 'Not Good' 

:-)

And it seems to be exponentially increasing from generation to another.

That's reminding me the French proverb " Too good too stupid "

Unfortunately, 'goodness' and "kindness" are often interpreted as 'weakness'.

They think that a person should always be tough.

Because there are many people who will/does take advantage of your goodness.