That's a good question. I guess it is because in mph, m does not mean metre and mph predates the introduction of the International System of Units, which anyway some countries have not adopted as the mandatory system or, as in the UK, adopted it with a few specific exceptions, but we are talking about road signs in Thailand in this topic, I believe.
Fortunately, though, it does not happen often that a space rocket explodes in flight because an engineer mixed up inches and centimetres. I have read about only one such incident so far, in the USA. I wish I could find that news article again.