Recruiters don't get paid if you don't get hired. They are not incentivized to get you the highest salary, they're incentivized to get you hired.
Not the case for internal recruiters.
It doesn't matter to them whether you make +-10k$ at the end of the year. It's about the same commission for them, they just need you to accept and move to the next candidate.
If anything they are competing on price and selling you for cheaper makes it easier to place you.
Not to mention that if they only have positions and companies that pay poorly in their portfolio, they're not gonna say it.
You sign at 100k, they make 20% so: $20,000
You sign at 110k, they make 20% so: $22,000
No deal, they make 20% so: $0
Do you think they have much incentive to make that extra $2k, while introducing more risk, and not spending time on the other openings they're working?
Why? Two reasons:
1. Because they are salespeople. In sales, you never leave money on the table if you can avoid it.
2. Good engineers are really hard to find, which is why the companies that use external recruiters do so in the first place.
Oh, and yes, external recruiters will negotiate salary on your behalf.