Actually, that's incorrect. There are a few who still do take direct submissions of manuscripts. But as been the trend in the industry, they won't be for very much longer. And you're right, the agents do act as the gatekeepers for the BPH's.
HOWEVER, there is yet one more exception, and this was brought up in other places as well. The BHP's who are only taking unsolicited submissions from agents at this time are also doing author head hunting.
And by head hunting, I mean they're watching the book circles for authors who are self published, POD, and PTP, as well as some authors who are published with small houses like myself. So what are they looking for? Successful authors. Simply put, if you can succeed at self publishing, POD, or PTP, and even SPH to some degree, they're interested in you.
And here's the general breakdown of success based on my own research. The actual numbers will vary by publisher, but this does act as a reasonable guide to show you how they do their research. And these numbers are based on the minimum total sales within a single 12 month period required to elicit the listed response.
20,000+ books sold. Publishers will put you on a "watch" list. If your sales remain steady or increase, you may get bumped up to the next level. If they taper off, you'll get dropped.
100,000+ books sold. Publishers will put you on an "interested" list. It's a type of "short list" for unsigned authors they may be interested in contacting in the future based on your sales record. In some special cases, depending on your genre, and their needs, some publishing houses may come to you and query you about publishing with them as you're a proven success. If they do, read the fine print, make sure the contract is good (if not, negotiate one that is good), and then sign.
250,000+ books sold. This is best seller status. If you can sell that many books in one single 12 month period, you'll have publishers beating down your door in order to try and sign you. I say that because, if you're self published, POD, or PTP and you sell this many, then you're a proven seller and they see no risk in signing you.
500,000+ books sold. Same as previous, but this only applies to you if you're published through a small house, as they've got some extra marketing muscle to help your book succeed, so the BPH's take that into account when considering you.
Anywho, those numbers are based off my observations and are dependent on the BPH in question. But either way, it's a good goal to shoot for.
Also, as a side note, some Agents are going to this method of scouting now too, where they will not accept any unsolicited queries for books, and instead go head hunting based on the total book sales of self published, POD, and PTP authors, as well as some SPH's.
I honestly think that in 10 years or so, the only way to get into a big house, sign with an agent, or even get into a small house in some cases, will be to first self publish, and then prove yourself as an author. Now I may be wrong, but I'm willing to bet that's where they'll both go in the near future.