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.

EOL doesn't matter - in use and in standard matters. Also the look depends on local conditions – the font. That's not what designers are keen on.