Sadly, vending machines are filled with mars bars and not celery stalks. Cities are overflowing with McDonalds' and Pizza Huts. You can stuff 2000kcal worth of fast food down your gob for under a tenner of whatever currency. But if you want a healthy salad, you'll be paying a premium. I feel like there's a business opportunity in healthy fast food, but as far as I can tell there's no such thing, at least not in any of the cities I've lived in.

Healthy fast food is not that appealing (to current consumer preferences).

Tetraethyllead was added to gasoline to reduce knocking. Unleaded gasoline wasn't appealing to consumers at all, but the market for that is now dead and consumers continue to fill up their tanks. The evidence for its harm was overwhelming. The evidence for fast food being harmful is a lot less overwhelming at the moment. Maybe once that becomes clearer, consumer demand will shift, hopefully without requiring legislation?

I don't see the analogy. Evidence for fast food being less healthy than a salad is pretty overwhelming, consumers don't care and choose fries anyway.

Lead pollution harms everyone, not just the person who burns it. And, lead isn't really needed to avoid knocking. If you want consumer preference to change you need to make a salad that is as delicious and cheap as fries.