Because as you know with F1 track position is everything because unless you are a second or 2 a lap quicker its near impossible to overtake.
Run the car light at the start, get it on the front row or very close to it and either keep up with the car infront (instead of losing ground each lap) or if first place pull a couple of second gap, pit early before anyone else so that you get out in clear air so not to get held up and then run a longer middle stint holding up anyone behind you that you can (as Webber does in most races) and then pit the same time as everyone else on the 2nd stop and race to the flag.
Webber tends to run heavier than most, he might have ok track position (borderline top 10) but doesnt have a chance of catching the cars infront because he is so heavy. I'd rather see him qualify at about 5th every race, then either keep up with 4th or hold up 6th whilst on the lighter fuel load.
Unless your car is quite abit faster than everyone elses the races are pretty much set in the first 15 laps due to track position.