That's probably because they require every employee to make contact with every customer they see. Impossible to get anything else done when you're doing your job in a store full of customers.
When they first started doing this I avoided going into Home Depot because I couldn't move a yard without some employee asking if he could help me.
Very different here in north Jersey; I remember years ago there would be greeters asking what you needed, but that was a long time ago.