Illustrator started as a professional print-only application. It was made for professionals creating pieces designed to be fed to an imagesetter and put on press. It was a print production tool and really not much more. There was no internet when Illustrator was created. No mobile devices. No UI development to really speak of. So, Illustrator was built on Adobe's new postscript language core. At version 10 things were changed and it's been a PDF core since then. SVG has never beed a primary concern for Illustrator.
The problem is SVG has been an infant, niche, technology for most of Illustrator's life. I wouldn't be surprised if there wasn't a time when Adobe contemplated dropping all SVG support from Illustrator due to lack of interest. It has only been in the last 3 to 5 years, with the advent of iOS and Apple's unyielding push to HTML5 that SVG has become more prevalent and its virtues more widely recognized. Remember Macromedia (and as of 2005 Adobe) had Flash. That was the go to app with SWF rather than SVG before Apple decided not to incorporate Flash into iOS. Apple effectively killed Flash opening the door for SVG.
Adobe, being a monster of a corporation with the mentality to fit that, is always 5-7 years behind current trends. So, SVG support in Illustrator is nowhere near what lighter, smaller, more nimble, application developers can support. You'll have to give Adobe a couple more years to catch up to yesterday. :)
Also realize that compared to other, similar applications, Illustrator is a monster. It doesn't even preform as well as other Adobe applications. It's really not multi-core aware and never even touches a GPU. Out of all the Adobe applications Illustrator is (at least here) the slowest of the herd. Because it's so old and has had so many features woven in and out of it's base code over 20-25 years it's very prone to the "make this change and that breaks" issue. It took Adobe 3 years to get Illustrator to a 64bit aware state. That's 3 years after almost all other Adobe apps were 64bit aware. Expecting Illustrator to support SVG features developed within the last 3 years is a dream that will most likely never happen.
If you really need good SVG support, I'd suggest Inkscape. Unlike Illustrator, it's built on an SVG core and therefore tends to support more SVG features. Of course Inkscape doesn't have the editing versatility of Illustrator, so it's a trade off. You could always work in AI, then save as EPS for Inkscape and refine in Inkscape.