This is because Swiss chard is a relatively new introduction to India, and like many other foreign vegetables, it is often simply referred to as a type of "foreign greens".
You might also encounter other variations like:
* சீமை பசலை (Seemai Pasalai): Meaning "foreign spinach"
* சீமை பழங்கீரை (Seemai Palan Keerai): Meaning "foreign leafy greens"
It's important to note that these are general terms, and depending on the region and local usage, there might be other names used for Swiss chard.