The job of a literary agent is to sell their clients’ work.
Good literary agents are paid a percentage of all the sales that they make. They don't get paid until their clients are paid, at which time they receive a commission from that payment: usually 15% on home sales, and 20% on foreign sales.
As agents’ incomes are tied so directly to their clients’ successes it should be in every agent’s interest to secure the best deals that they can for every single one of their clients, and to consider both the immediate and the long-term value of every contract that they are offered. In order to maximise the value of the contracts that they negotiate, agents need years of experience and a network of excellent contacts within publishing, along with a list of clients who regularly produce high-quality, marketable work.
Some literary agents prefer not to work on commission. Instead, they charge an up-front fee, an annual retainer, or both. As fee-charging agents get paid regardless of how much or how little agenting work they do, they have to collect as many fee-paying clients as possible in order to maximise their income, regardless of the quality of their clients’ writing. They have no incentive to establish good contacts within publishing; to consider what's best for their clients; or to even attempt to place any of the books on their list. And, as it costs money to make submissions and they’re not likely to make many sales because much of their clients' work is of questionable quality, then the only submissions that a fee-charging agent is likely to make are to vanity presses.