Big G

This G is with us since last centuries. Of course, it is Newton gravitation constant. Traditionally it is written using a capital G, and yes! it is a big G!
But this one, every universal constant in well known and calculated accurately. Why is this? G can be only measured by direct calculations. Since gravitation force is the weakest one -the force the Earth attracts an apple can be overcome with your hand, G is elusive... Scientists have made efforts to get its value as much accurately as possible.
Newton did not measure the constant but henry Cavendish years later (1798). His instruments gave a G value of 6.754*10^-11m³kg^-1s^-2.