Because a chimney sweep will naturally be the person who services the appliance and chimney after they install it.
Because of this 'field experience' they are the best at matching chimney systems with wood burning appliances and knowing what type of set-ups will not perform well or even which stoves are poor choices.
So, I suggest that you look at the advertizing of the hearth dealer you are planning to buy from. It is astounding how few stove sellers have little or no experience sweeping chimneys and servicing appliances. It is no question that hardware stores are the least qualified in this regard. But, there is no excuse for a specialized retailer to not genuinely service what they sell.
Please consider, if you are buying from a hardware store- simply because of the price- the product may not be as good in terms of quality. Also the service before and after is practically nil.
Can you really trust that the guy behind the counter will sell you what you actually need? He will never see you again, so why will he care?