sightly is a thing... Something that looks good.

This is very much "I am 14 and this is deep" territory.

Adjectives describe. By using them, we are emphasising a quality of a given thing. That does not make it the "default state" (a problematic concept) of that object, even if it is a desirable quality.

The "default state" of food is that it is edible, ie. that it can be eaten, as food is defined as that which is edible.

there are no corresponding adjectives for the senses of sight and touch

Visible. Tactile.

Noisy

Even by OPs logic, "noise" is not one of the senses. Audible is the correct word here.

https://en.m.wikipedia.org/wiki/Problem_of_universals

The default touch verb is bad, because nobody has ever used the word "touchy" to mean something good.

When you say someone is a real looker, it means they're attractive, so I'd say the default state for sight is appealing.

My thesis brings all the boys to the forum.

And they're like, "This makes us a quorum."

Damn right, this makes us a quorum.

I could teach them but I might just bore 'em.

Basing life philosophy on English grammar...lol

But there are words for those two thing, even if they're not used commonly.

Sightly is something that is pleasant to the eye. Sure, it's inverse "unsightly" is more commonly used today, but it is still a word.

While not mainly used to refer to the sense anymore, Touchy can refer to a body part that is acutely sensitive or irritable.

Farts must be delicious 🤤

You're definitely describing a fart.

He's totally describing chickens. Aside from the invisible part.

The person is not wrong about Greek Philosophy. Have been reading some of Plato’s works and he does have a bit of an absolutist way of presenting things that sometimes fails to address the nuance. The things that he is right about though transcend time and are eerily relevant to our current circumstances, so his thoughts on him an nature tend towards accurate. On this example, Plato would probably script Socrates explaining to Glaucon, and indeed does, that vision is tied to brightness and darkness and that somewhere in the middle is where you’d want to be because brightness is brought by the sun, which is hot, but you can be tool dark and cold in a cave. He’s then be ADHD and explain that the cave is a metaphor for our knowledge and since light lets us read, we lack much knowledge when being in the dark. Glaucon would then quip that people can still talk in a dark cave and that their voice might even be echoed and Socrates would probably say that without the warmth from light, the soul won’t listen to the loudest voice. It goes something like that.

Screaming invisible cheese?

A gas being forced through a pipe that's smelly yet sweet-tasting?

Why do we park on a driveway and drive on parkway?!

It does match one phenomenon. Survival. Noise is important for listening for predators, smell let's you know if potential food is rotted and our sense of taste helps us distinguish between calorie dense food (sweet), salts and micronutrients (salty), acidic foods (sour), poisons (bitter) and protein (umami).

Bro forgot disgusting

Fuzzy and pretty...default object I propose is within the fungi family of life.