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Because ultimately the finish line is an invisible plane defined by the finish line camera, not by the line on the road (which is about 5cm wide anyway) or anything else, and the 10 000 fps time resolution of the camera is sufficient to differentiate. Same for athletics as far as I know. I don't know about swimming, but presumably imaging is more difficult.
chez_jay-
The finish line is mandated to be 4cm wide, painted black on a wider white strip, and the winner is the rider whose front wheel breaks the plane of the leading edge of the black line first. That's what the rules say about who the winner is, no mention of a camera. Unless that camera can be reliably aligned to that plane with mm accuracy making a call that fine just isn't possible. I don't believe the camera can be that accurate, or even that the line is drawn sufficiently straight.
Malaconotus
Just watched highlights and that result's bollocks. It's a dead heat. Olympic swimming measures to hundredths, not thousandths of a second because you can't build a swimming pool accurately enough. Lanes get longer and shorter by several millimetres just due to temperature variation. There's no way the road is built, the line is painted, or the gantry is erected with sufficient accuracy to call that. The road is even curving slightly. How can you even decide which line is perpendicular to it with 6mm accuracy? EBH was robbed.