I agree that it’d be nice to see more games open their assets up for others to benefit from. I would point to a couple of things though:
Games are in kind of a weird space where they use huge amounts of media compared to most other art forms. Taking an asset from a game might just mean taking a tiny piece of the whole, but that piece is still usually a complete work of art on its’ own. If I had to guess, that might be part of why it’s less socially acceptable. Not to mention that in the case of Mario, we’re running into a major global brand. Most big media companies are protective of their properties regardless of medium, just look at how Disney acts for example.
it felt overwhelming that the guidelines of copyright infringements are very strict, to the point where someone using an open source art could get some backlash for simply not crediting all of the assets used on their game
This might just be me, but I haven’t personally seen anyone get that sort of backlash over open-source assets. More importantly, as someone who helps run an open-source modding platform… there is a very valuable reason for those rules when it comes to open source. I’d argue that these licenses aren’t really to benefit the creator, but to protect the remixer.
In most spaces where the use of such licenses when sharing isn’t common, a creator can potentially change their mind and sue someone for using their work. Will they win? Who knows! But you can see this effect in music–some smaller bands have sued big ones long after their work was re-used to try and “get a slice of the pie”, so to speak. With a contract in place that explicitly lets you use the work, that’s not really possible.
Ultimately, I think many small artists are fine with pieces of their work being re-used so long as you talk to them and show good faith. I make my game assets free to use with a couple of basic restrictions, but those restrictions mostly exist to prevent people from plagiarizing my work; if you took a small asset from a game of mine but didn’t give credit, as long as you weren’t doing anything nefarious I’d probably just ask politely in private rather than stirring up a big fuss.