You said it yourself, "a comma separates stuff that belongs together". The integers. I can type 27000 and its valid, I can space them integers with a comma 27,000 they belong together. Decimals are different to integers, so they are marked with a period, like the end of a sentence (of integers).

You can argue either way honestly, but more of the world use periods for decimal notation. So it would make more sense if we just adopted that (never going to happen though).