Because it's an excellent investment, socially speaking.
Because investment in the arts (I mean serious investment. Not like USA) has worked pretty well for some countries.
And we're talking all realms of unrestrained creative effort here. Science, technology and stories about dragons. Opensource software as well as basement watercolorists. This is where the shiny new legos of our society come from.
I think that's just misunderstanding cause and effect. It's more that nations "invest" more in art as they get wealthier. It's not that they pay a bunch of people to do whatever they want and tada, the country becomes industrialized.
Perhaps we should invest infrastructure, systems and institutions that give people more free time, flexibility, freedom, and inspiration so they can create good art rather than "investing in the arts"--whatever that means