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Isn't this because most businesses don't have phone systems now (or that awful computer based one that no-one uses) so people use teams/slack as an alternative to the phone and that initial 'hi' is really to be understood as 'are you online now and able to help me out quickly' - the person can't be arsed typing their full question because they want to know first whether or not you're there and able to help.
Mate! Come on just tell me what you want.
I wish I had the patience to keep it going with endless hi + pause responses.