I thinks the salt used for melting the ice, the high variations of temperature, the surface being wet non stop for weeks when the snow melts and the frequent snow plowing (which is a huge metal shovel that scraps the pavement) contributes a lot to the fading.
I fucking hate cars too and I agree with you they are completely inefficient. But paint on pavement in cities where the temperature is near 0°C a good part of the year is also inefficient.
I doubt the plowing is a big factor since we actually plow our bike lanes much less frequently than other cold countries like Finland and they paint their bike roads. Then again, you have a good point about paint being invisible because it gets covered in snow.
In Finland they use lamps above bike lanes to add colour and signage over the snow. I just think the gray asphalt makes it feel like the area is still for cars. That psychological effect is a huge part of good street design.