Maybe because you can't guarantee that even if an OS has emoji support that it supports the latest version of emojis.
I run into this every time my wife updates her phone. She sends me text messages with "Lookit the cool new emojis!" But since I haven't updated mine, they came through as missing icon glyphs.
I think google renders them in a raster format. Whatever they use to do that doesn't support emojis.
It could be by design, by rendering any text in the svg icon google is making a design decision for you. They're choosing a font, or a specific emoji type. Maybe they want to avoid the problems that would ensue.
