I suspect that heat packs, which might be adequate for warming hands for a short time, just don't pack the sort of heat output or duration to be a primary heat source in a vehicle which is poorly insulated. They might be useful for hand warming and small jobs but not as space heaters.

There is also the issue of cost. 120 hour multi-wick candles can be had for less than 16, sometimes around $10. An ammo can full of chemical heat packs is going to be pricey. Even more so when you have to replace them as meet their expiration date.