It’s typing. It takes literally… what, two seconds to type that? It’s really not that hard. I don’t see what the big deal is.
Some people care and some people don’t. When it’s people I’m not comfortable with or familiar with or who I don’t know if they care or not, as a general rule of thumb, I default to being formal and professional until I know it’s okay to be otherwise. That’s just the safe bet. Some people will care and some people won’t. But for the people who do care, it can be a big deal (as it was with the OP’s professor). Many people won’t care, but it takes two seconds. It’s not the end of the world. I don’t see why it would bother anyone so much. It’s like saying please and thank you. It may seem tedious, but it’s nice and it takes a second to do. What’s the big deal?