Almost everything you need to create vector art, SVG doesn't support.
Multiple outlines in a single shape? No. Varying thickness in an outline? No. Rounded corners on arbitrary vertices? No. Non-destructive boolean operations? No. I'm not even sure SVG supports paragraphs.
Many of these Inkscape implements as live filters, which are saved as SVG extensions in the XML .svg file that nobody but Inkscape can properly load.
SVG is ridiculously bad as a creation format. It's a good format to export to, but as a backend and it's just insane. It's like using a single PNG file as a backend for your multi-layer 128bpp raster project.
I use Inkscape a lot but I can't help but notice that the best vector art illustration come from Affinity Designer, Corel Draw, and Adobe Illustrator. If you compare the quality of artwork made with proprietary tools to those made with Inkscape, it's very clear that Inkscape severely limits what artists can achieve. You can easily create complex illustrations in other tools that would be a nightmare to manage in Inkscape. Just compare how you clip something in Inkscape to how you do it in Affinity. It's ridiculous how different the two workflows are.
Do all the others (AD, CD, AI) use some proprietary format that makes their life easier? Is there no better alternative to SVG on the open source side of things?
There’s a conflict between what is needed to simply display images, and what is needed for an ideal editable document.
A web browser doesn’t need the complexity of non destructive Boolean operations, but an editor program does. It can all be exported to SVG at the end once it doesn’t need to be edited anymore. Things like variable thickness outlines can just be turned in to filled Bézier curve shapes that happen to be the shape of the outline.
You are doing the mistake of associating open format with standard format, which really are orthogonal concepts. Open format means the format specification is published under an open license. A standard format is one whose specification is maintained by a standards organization/body/consortium. It happens that most open formats are or end up being maintained by a standard body out of convenience[1] and because people often publish them in the open with the hope it will also be used by others but it doesn't have to be. You as developer of application foo can publish the spec of your .bar format on foo's website under an open license and do the fuck you want with it while not being limited by the potential slowness of a standard governing body.
[1] mostly to avoid multiple incompatible forked versions of the format being used with the same name and confusing others.
The issue with using SVG is it's not meant for use as data exchange in vector graphics editors but for web publishing.