An extra 5% on $100k to you is $5000 but for the agent it's $250. They are not aligned to get the best price.
It doesn't mean they won't try hard but keep in mind $5000 means more to you than 250 will to a company.
I wind up handling negotiation past the intro and initial position details myself anyway it's something I find best done by me as primary concerned party and apparently so very few people negotiate at all so there's usually wiggle room.
That's why I wound up using the offer they found me to push one I liked more locally because you're right they're only so aligned with me and being able to fall back on that offer let me likely top out the comp range at the local place.