NYC does have a number of dedicated salt-spreader/plow trucks [1].
As for why they use garbage trucks to plow, I speculate that it's an issue of scale. Buying a special purpose plow truck to replace every garbage truck that currently does double duty would leave you with a pretty significant/expensive fleet of trucks that are sitting idle most of the time (not to mention taking up space, which is at a premium in NYC).
As for why they use garbage trucks to plow, I speculate that it's an issue of scale. Buying a special purpose plow truck to replace every garbage truck that currently does double duty would leave you with a pretty significant/expensive fleet of trucks that are sitting idle most of the time (not to mention taking up space, which is at a premium in NYC).
[1] https://www1.nyc.gov/assets/dsny/site/about/fleet