Start running down the list of classic famous artists and you may be surprised at how many wouldn't have met those criteria at the time when they created their most important work.

To be fair... a lot were. If anything, most of the artists we still remember from before the modern era did their best work under wealthy patrons or a purpose (David's Death of Marat and Oath of the Horatii both being pseudo-propaganda for the French Revolution).

The view of the starving artist doing their best work for free is compatible with our modern, post Industrial Revolution views but largely inaccurate.

Should artists be exempted from the basic requirement of being able to support themselves that we expect from people of other professions?

If the bar for "can support himself" is suitably low, someone could be an artist, immigrate to the US to work as an engineer or accountant or nurse, and be an artist. Once he can support himself through art (and is likely a citizen), then be a great artist full time.

You have a really sad view of what "art" is, if you think "oh, he can just be a <insert equally time consuming profession>"

For a period of time, I absolutely do think someone can do some other career in addition to art, rather than receiving public assistance.

What is the point to making self-sufficiency an externally-enforced requirement?

If are are going to be deprived of the means of subsistence by state force, we shouldn't expect them to be able to support themselves.