NC covered works trick people into investing in a dead end
Free licensing lowers the barrier of entry to creating cultural works, which unlocks a dynamic where people can realize their ideas much easier - and where culture can actually live, creating memes, adjusting them to new situations and using new approaches with old topics.
But for that to really take off, people have to be able to make a living from their creations - which build on other works. Then we have people who make a living by reshaping culture again and again - instead of the current culture where only a few (rich or funded by rich ones) can afford to reuse old works and all others have to start from scratch again and again.
Sharealike licensing gives those who allow others to reuse their works an edge over those who do not do that: They can access many resources early in their career which allow them to produce high-quality stuff without needing to pay huge amounts up front. And they hone their skills in working with free stuff. So when they become good enough that they can work in art for a living, they are deeply invested in free culture, so they have very good reasons for also licensing their new works under free licenses.
As a real-life example for the dynamic of free licensing, I’ve been working on a free tabletop roleplaying system in my free time for the last 10 years. For 3 or 4 years now it has been licensed under the GPL, so we could use images from Battle for Wesnoth in our books. And 2 years ago, I worked together with another roleplayer to create minimal roleplaying supplements on just one Flyer - where only half the images were from Battle for Wesnoth, because a great artist decided to contribute (All hail Trudy!).
All this would have been possible with NC licensing.
But about 2 months ago a roleplayer from a forum I discuss at unveiled his plans to create a german free rpg day and I realized that our minimal RPG would be a great fit for that - but that I could not afford myself to print it in high enough numbers and good enough quality to reach many people.
So I worked on the design and text to polish them, and when I was happy I started a 4-day fundraiser to finance printing the RPGs. Within just those 4 days I got over 200€ in donations which allowed me to print 2000 RPGs in great quality along with supplements and additional character cards which made every single RPG instantly playable - instead of 1500 RPGs with only one card so people would need 3 RPGs to actually play.
And this would have been plain illegal with NC material.
It is not yet “making a living with free art”, but it is a first step out of the purely hobby creation into a stronger dynamic. One which allows us to bring 2000 physical RPGs to people without going broke - and more importantly: One which started small and can grow organically.
An RPG might not be the best example here, because tabletop RPGs are notoriously bad for generating money. But it is the example I experienced myself.
As an example which might be closer to you: Imagine that you created a movie with free music and other material from free licensed works. Imagine that half of the visuals you use could have already been created - maybe for some other movie. By using free stuff, you could save half the effort for creating the movie.
But if that other stuff had been NC, you would not be allowed to start a fundraiser for getting it to blu-ray quality - at least not without replacing all NC parts, which would have added a high cost to be able to increase your outreach. Likely it would have been a blocking cost. It would have been easier to just create a new project than to polish the one you have to reach more people.
And polish is what allowed me to move the RPG from just being a hardly readable PDF to a work I can look at with pride.
To wrap it up: Free culture - just like free software - allows people to take little steps into creating culture and to move organically from just being a hobby artist towards making a living from their work - and spreading their work to many more people.
And NC covered works trick people into investing in a dead end, because they can never move beyond being a hobbyist without huge investments which bring no other benefit than recreating what they could directly use when they did not try to make a living. It’s like learning to use Photoshop and then realizing that you aren’t allowed to earn a little extra by improving wedding-images without shelling out 3000€ for a creative suite license. And that means, that you can’t move in small steps from a boring day job to a professional creative life.
(written in reply to a question from Keith, one of the makers of Software Wars, a movie about free software which is trying to fund going to a high-quality blu-ray release at the moment)