Why don't most ice cream recipes include salt in the base?
I can't read the minds of all the recipe authors, but I do have a couple ideas.

I've found it's definitely easy to overdo salt in ice cream. A very small quantity goes a long way. In most cases you don't actually want it to taste salty, you just want a tiny hint that people will only notice as improved flavor. The quantities are small enough that it'll be hard to provide accurate measurements, so you're stuck saying "a pinch" and hoping people don't use too big a pinch.

It's also just not the most obvious addition to most people. Ice cream can be pretty darn good without it. While everyone's used to salt in baked goods, as you said, ice cream recipes often don't include it, so there's not too much learning by example going on. We certainly don't usually put it on fruit! So if you're looking at recipes online that people have come up with on their own, there are decent odds that the author just didn't think of it, even if it would've actually been a nice addition.

Finally, just to clear up an oft-repeated misconception: in these quantities, salt is not going to mess with the freezing point, just like it won't affect the boiling point of pasta water. It takes a ton of dissolved salt to significantly lower the freezing point, and we're making ice cream, not seawater slushies. (Even if you did lower the freezing point, it'd probably be a good thing, since homemade ice cream is generally too hard, not too soft.)