Because they are not just barely lighter (on an RC which I know the weights) Further the overall weight is important but the distrubution is equally important as the further from center the weight loss is the higher percieved change.
On some bikes, aftermarket aluminium wheels are actually heavier than the OEM wheels.
The rims of the stock wheels and aftermarket wheels are essentially the same weight - they're both aluminium and forged material is denser than cast material. Any real saving is in the hub design.
The weights you've listed for the rear wheels are without the cush drive. My Marchesini three-spoke mag rear weighs 4.051kg (9.034lbs) with just bearings, spacer and valve. Are you sure 7.76lbs is correct?
Does that 8lbs aluminium front include the bearings and spacer?
A stock 929 front wheel weighs 4.144kg with just bearings, seals, valve and bearing spacer (which I make as 9.24lbs). The discs are another 2.65kg, bolts are 0.120kg and the tyre is around 4kg (new) depending on brand.
The 954 wheels are slightly lighter than the 929 wheels (due to the cut out sections) but I haven't weighed them to know by how much. The '04-'07 1000RR rear wheel is over 300gms lighter than the 929 rear wheel and everything that attaches to it is exactly the same part as on the 929/954.
Assuming the 8lbs (3.587kg) is with bearings and spacer, runs the OEM discs and is the same pattern for the SP2 and 929/954 then the forged aluminium front wheel saves you 557gms (1.24lbs) most of which is in the hub Less on the 954 as the OEM wheels are already lighter). Certainly a nice saving but is it worth $1000? On the SP2 (4.305kg) you could put a 1000RR front wheel on and save perhaps 200gms for just a couple of hundred dollars.
Assuming the 6.04lbs (2.709kg) is also accurate, the mag wheel saves you 1.435kg for around $1500. And now the rim is two-thirds the weight of the aluminium wheel purely because it's magnesium - definately better bang for buck.