Best offer does not mean highest offer or first offer received.
Sometimes likelihood of closing is more important than price.
Sometimes speed of closing is more important than price.
An all cash offer is more likely to close.
An "as-is" offer is more likely to close.
Many factors can make a lower offer more favorable than a higher offer.
Is it the listing agent's responsibility to conduct a multiple offer scenario?
Some buyers get upset or too stressed to participate in a bidding war. A bidding war may drive the best offer away for good, thereby driving away the offer the seller was content with or the offer the seller really wanted to accept.
There are many variables. The reluctance of the seller or the listing agent to conduct a multiple offer scenario is not necessarily a sign of laziness or irresponsibility. On the contrary, it may be a sign of an astute Realtor who knows exactly how to deliver the results the seller is wishing for.