The unpalatable truth:
1. Because most aircraft now weigh considerably more on the taxiway than they do on paper, and this suits the operator's bottom line.
2. Because nowadays there are very, very, few performance related accidents involving big aircraft or events where the fact that a big aircraft was slightly overweight was critical, and so...
3. Regulators and investigatory authorities never weigh big aircraft after accidents (though they often comment that small aircraft were overweight, which they often are).