This need not be true: you can put objects and viewboxes wherever you like in SVG. <svg viewBox="1234567890 1234567890 100 100"> is perfectly valid.

Many years ago (Adobe SVG Viewer days) my Dad generated SVG files from geo/map data and at first used the original coordinates or something, but float error made it untenable—some viewers would end up quantising some things (e.g. the shape, the viewbox, individual nodes, even once you applied a transform or SMIL animation to them, the hover hitboxes—I don’t remember the particular details, but it was a few things) so they’d end up off by as much as hundreds of pixels in the most extreme projections. Therefore he ended up rezeroing to work around this, transforming into a more friendly space before producing the file.

In the context of SVGO: really, the whole decimal places thing is stupidly bad, because decimal places can be of wildly differing importance in different contexts, and it wouldn’t be that hard to figure most of them out. In some situations a third decimal place will be more valuable than another number’s hundreds place. And SVGO’s approach is particularly bad because it treats each number individually, not attempting to compensate for the error it introduces, and so the presence of fine detail can easily completely destroy things due to untreated cumulative error.

I wouldn't expect SVGs that require double precision to render anywhere properly. Most renderers these days use 32-bit floats simply because it's easier to get things to render on the GPU that way, so at very large or very small scales things tend to break in fun ways.