They're still active participants in traffic. Someone who is waiting for the signal without the distraction of a smartphone will have seen the flows of traffic commence and stop in the other lanes and the roads that meet at the crossing, and will go into the crossing more relaxed and with better situational awareness than the person who just got jarred from chatting with someone about whether to get take out, and “oh, could you drive past the supermarket to get some wine?” into moving because of the honking behind them. They are also much more likely to notice irregular or otherwise unsafe behaviour by others around them (like a kid on a BMX dashing across at the pedestrian crossing too late to beat the lights).

Collectively, these moments of reduced attention cause way more harm than they should, all because so many people can't leave their damn phone alone while operating a vehicle in traffic.