As an example, suppose your house could sell for $750,000 immediately with you paying a 6% commission of $45,000. Or, suppose the same house could sell in 6 months (after a lot of showings) for $800,000 with you paying $48,000 in commission. The $50,000 extra you'd make on the sale is really big for you. But the extra $3000 the agent makes for months of extra work and waiting around is nothing. She'd rather take her $45,000 after selling it cheap after 1 week and then move on to sell another house.
Isn't that the normal dynamic?
EDIT: I've heard that you can write a listing contract so that the agent's fee is tiered. So, for example, they'd get 1% if they sold it for $500k because even a "for sale" sign written in crayon in front of the house would sell it, but then 6% on the amount between $500k and $750k, and a juicy 18% or whatever on the price above $750k. But does anybody actually do this?