In what way were their interests aligned with yours? (this is not a snarky question. usually in real estate their interest is to make the deal happen asap, which doesn't align fully with your interests)
But even aside from that, they make more money the larger the deal, so they work to get a good price. They also feel the pain if the deal falls through, so they have incentive to keep the deal alive and both sides happy.
The broker isn't perfectly aligned with me, but of everyone else involved like lawyers, accountants, and the buyer, the broker is by far the most aligned with my interests.
In real estate brokers typically get around 2.5%, so even if they find a buyer that is ready to pay 100K more, it's only $2500 extra (so probably $1500 after taxes). It's not a lot of money esp if they can invest that time and energy in parallelizing other deals. Time is of the essence in such transactions.
And then of course a Buyer side agent's incentives are aligned exactly opposite to the buyer, but even if they would want to push for an early transaction above anything else.