About half the deals that I do are with a mortgage broker that I recommend. The other half is a toss-up. Meaning I never know who I am going to get to work with. Yes, work with. A lot of buyers think that they will be the only ones working with their lenders, however as a real estate broker, I put in a lot of time with them as well. It's my job to make sure that they have all the documents necessary for the sale, as well to make sure that our mortgage approval deadline is met.
The deadline that was created in the offer is of utmost importance to everyone involved. Sellers are already nervous, so to ask for an extension could potentially kill the deal. They are not obliged to extend. It's up to me, if I am in that situation, to convince the sellers and their real estate broker that we are still on track to get the mortgage approval, even though we haven't gotten it yet.
One of my jobs is to stay in collaboration with the listing broker. If I keep them in the loop every step of the way, they will be understanding of what is happening if I have to ask for an extension. Normally it's in the best interest of the seller to continue with the offer rather than drop it for the next one. However if you got the accepted deal while in a multiple offer circumstance, the situation isn't stable. The seller doesn't want to lose time dealing with you when they might have another buyer who has been bugging them for a chance.