The problem is that a filament is a very small point emitting the light, so the reflector and lens produce a beam with a nice sharp cutoff. An LED is much bigger and the result is half of your dipped beam now ends up pointing at oncoming traffic. Therefore they’re not legal.
There is a Phillips led bulb that is approved, but its a lot of money. Might be worth it on a new car you expect to keep for a long time if the alternative is buying nighbreakers.
There’s some beam shots here of what happens if you juat use led bulbs off amazon/ebay/wish/aliexpress :