Because that’s how the web renders type? (No.) Does iOS or Android behave that way? Maybe I’m missing something.
Also, how often is this fit within an actual use-case? Having a consistent baseline rhythm is more noticeable, and meaningful, in multi-column text layouts—something that doesn’t come up often on the web, and less often on smaller devices (native apps seem to be your target market).
Supporting flexible grids, and better guides, seems like a much bigger win—but maybe enabling designing for multiple devices isn’t a priority?