Because car "low beam" headlamps are required to be focused and controlled, whereas most bike LED lights are like flashlights, with a conically-shaped beam with a lot of spillover that not only does nothing to light up the road, it goes into the eyes of oncoming traffic users (cars, cyclists, everyone). Essentially, you're running around with "high beams" on, and if you're a driver, you know how annoying that can be.

If you really want to see what a relatively small amount of lumens can do when properly shaped, investigate the lights that are StZVO-approved. Examples include the Schmidt Edelux and Philips Saferide (video). Especially for the dynamo-driven versions, the lumens are restricted because power input is restricted; however, these lights still perform brilliantly (no pun intended) simply because they don't waste about half their energy on lighting up trees and blinding oncoming traffic.