Why the kilogram is not defined? [duplicate]

Possible Duplicate:
Why do we still not have an exact definition for a kilogram?

I was thinking about SI units. I found the following definition for the base units:

• Meter: distance covered by light in 1/299792458 seconds.
• Second: duration of 9192631770 periods of the radiation corresponding to the transition between two levels of fundamental state of 133-Cesium.
• Kelvin: 1/273.16 of the thermodynamic temperature of water triple point
• Mole: Number of atoms in 0.012 kg of carbon 12
• Candela: [...]
• Ampere: [...]

I searched the definition of the kilogram and I found only this one:

Mass of the international prototype of kilogram.

Why such definition? It is impossible to well define the kilogram? Why?

By my point of view the mass of the prototype will change a little with time. Is this effect considered? And what about definition of mole: it is based on kilograms, so also mole definition is "impossible"?

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marked as duplicate by dmckee♦Oct 9 '12 at 17:04

I know this question is answered elsewhere, but the answer is that the standard kilogram gives a more reproducible weight than you would get from something like $\frac{1000}{12}$ of a mole of carbon-12. – Jerry Schirmer Oct 9 '12 at 16:51