What is a painting? The use of art includes making us feel certain ways -- but that's also the use of propaganda, for instance, which we'd not call art. — Moliere
Vietnamese propaganda posters are considered Art by some -- including myself. I think propaganda often makes use of art to portray a message. This point may make it easier to see where I am coming from in terms of conceptual art not being art. It is not that ALL propaganda and ALL conceptual art is not art, it is about the intensity of the Art elements -- one key aspect I refer to as 'moving' the subject.
Anyone dedicated knows that there are techiniques they use, intentionally or not, that play on human perception. There is always an element of 'deceit' (maybe too strong a word) in this. An instance of this woudl be how horror movies use low frequency sounds that cause all humans to feel like they are being watched. This is obviously useful if you are trying to induce a certain emotional response to the film they are viewing. An artwork has to draw the eye or ear and -- primarily -- the feelings of those exposed to it. If there is an area of sensory experience I am unsure of when it comes to Art it would be cookery. This I find hard to place within the realm of Art in the sense of Artwork. I think it is in areas like this that we have one term 'art' and another 'Art,' where the former is more in lien with the ancient Greek 'arete' rather than referring to something liek a painting. Of course, the problem is we can talk about the arete of the Art, or art of the Art. This is where I think the mongrel language of English causes confusion.
To get back to the whole issue of Drawing and Painting (got rather sidetracked there!) I do think the element of space comes into play quite substantially. A drawing is often much smaller due to the size of the tools and the application of material onto the paper. A painting has a much larger reaching scope.
An artist out to produce a work -- in any medium -- will often begin with pencil and paper; be this to write dialogue, capture a poetic moment or literally make a rough sketch. The daily use of a tool is not something we find readily in a paintbrush compared to a pen or pencil.
I think the main difference is likely in these elements. The size and scope, as well as the daily familiarity with both tools and materials used in the process. Paint can be made from many things, so there is more of a variety in terms of texture that does not readily lend itself to paintings. There is the fluidity of painting to and layering, that can be something form of ink drawing can produce but is not exactly prone to function in that way.
I do think with more impressionistic and abstracted styles there could be an argument that painters have adopted drawing techniques such as making use of hatching techniques in their own way. Saying that, more ancient forms of Art used such abstraction early on, so realism is likely the 'unusual' case historically for a number of reasons.