It's just one of those funny english-language things, I guess!
This
website (http://www.wikipedia.org/wiki/Collective_noun) says:Many nouns used are colourful, or
even fanciful; this originated in an English hunting tradition (of
uncertain origin) for giving poetic names to prey. (The phrase
"terms of venery" is an archaic synonym for collective nouns -
"venery" in this context meaning the "act of hunting"). For this
reason, most collective nouns refer to animals.
This tradition dates back to at least the 15th century . Many of
these original collective nouns are archaic: a "harass of horses"
doesn't seem to have been used much since the 1400s .
Interest in collective nouns has always remained high, and the
neologism of candidate collective nouns has been a pastime of
many writers ever since. Some have achieved an entry in a
respected dictionary, the vast majority have not. Some
collective nouns have been circulated on websites for
humourous reasons. In at least two cases (an "abomination of
monks" and "a court of kangaroos") some authoritative
resources allege them to be accurate, however research has
proved these to be spurious as well.
Some alternatives for collective nouns can be clearly traced to
the evolution of pronunciation in different areas (hence a "parcel
of hogs" and a "passel of hogs").
You can find lists of these collective nouns all over the internet,
for example, this website (http://users.tinyonline.co.uk/gswithenbank/
collnoun.htm).
Other languages have similar quirks, for example in Japanese
when you count things, depending on what you're counting you
use different words (e.g. you don't cound pencils, cars, cats,
elephants or sheets of paper the same way).
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