Multiple reasons, first, in Deuteronomy it is explicitly stated to not erase or desecrate his name. Therefore to add the - is a sign of respect, endearment and reverence , practiced primary by orthodox Jews. I personally maintain this tradition. I could just type Hashem I believe but I don't do so on a.regular basis. A similar comparison from the Christian perspective which I was most privy to as a kid, to "not take the L-rds name.in vain". So I avoided.ever saying "oh my G...". An old British teacher advised a student that it was considered an extreme curse, blasphemous I suppose. This stuck with me as a kid and I sought answers for this seemingoy innocuous saying of sort. I thus never verbally said "oh my G..." . Now, as a practical and theological matter, typing the name is akin to this offense. Deuteronomy states it in detail but I am too tired to search the specific. text.
