why do we call it a ton?

derrived from tun referring to a large cask/capacity, which the largest was supposed to carry roughly around the same weight.

TON IS A WEIGHT UNIT USED IN OLD DAY BRITISH SYSTEM, AND NOW IS NO MORE USED IN WORLD EXCEPT IN USA.

GRAM IS ANOTHER WEIGHT UNIT FROM ANOTHER SYSTEM CALLED MATRIX, COMMONLY USES IN MOST COUNTRIES NOW.

You're confusing a ton with a tonne. A tonne is a megagram. A ton is two thousand pounds. 

If you are so bored why not do some research rather than expecting others to spoon feed you the answer. It would relive your boredom and you would learn more if you did the research.

A ton is an imperial measurement and not a metric one. A million grams is NOT the same as an imperial ton.

In the metric system you use megagram in SI Units. However, while favoured by science and the best one to use to ensure consistency the SI system is not THE metric system. There are others.

The term tonne can be used but if correctly applying the SI Units metric system you would use one megagram / 1 Mg which could also be written 1 x 10^6 g.

Actually, it's tonne.

What sounds strange to me is that a fundamental quantity, such as the kg in SI (International System of Units) is thousand times (k) another quantity such as the gram

TonNE. If you knew how to spell it.... You also don't seem to know the meaning of mega. It would not apply to a gram. They are contradictory terms.

We call it a ton because the English started calling it a ton in the 14th century.