OK, so what you mean is, million and mega happen to share the initial, and trillion and tera happen to share too, but billion and giga happen to not share an initial.
Also, the metric prefixes are used cross language, and the ms, bs, and ts aren't the same cross language
Because it doesn’t scale. Giga is the metric prefix for “times billion”.
However, large (and small) numbers don’t end there. What about Quadrillion and Quintillion? What about milli and micro?