Anchoring effect. You're showing your hand in poker before placing bets. You'll never get a substantive increase in pay since the recruiter knows what you're currently willing to work for and that some small percent increase is still a win for you. Big jumps also usually means moving out of the range of jobs you're a "safe" fill for and that makes them view it as a higher risk of not getting paid for the effort trying to place you.

If you're not well networked and you're not enough of a rockstar to have companies approaching you out of the blue, your best bet is to control where that salary anchor is placed. There's hardly any way to verify your claim and you're almost always better off placing it at a believably higher position. I mean, companies aren't going to tell your their actual upper bounds for what they'll pay in salary, why should you tell them your actual lower bounds?