Currently all icons are stored in code as SVG path strings: https://github.com/wikimedia/wvui/blob/master/src/themes/icons.ts
Icons are then built using a Vue template. This means that no SVG files exist in the repo, and therefore the process for editing/creating new icons is a lot more convoluted. Also many code review tools support SVG rendering, so if there were real files, icon changes could be seen in diffs. Icons could also be previewed in the file system, loaded into SVG editors easily, and exported to other codebases.
If it is a requirement that the current SVG template is enforced, a simple linter could verify this (although it could just be enforced by convention in the meantime, as we have done successfully in OOUI).
Additional hurdle is adding SVGs to the lib needs another transformational step to single path SVG objects, which is developer unfriendly for readability (diffing) reasons _and_ more work when adding/changing existing icons. Icon addition is a common task with the still smaller icon collection of Wikimedia Design Style Guide and will occur often (possibly 3-6 icons per quarter, not counting the variants)