Wrong wrong wrong! Even if you don't want to participate in their way of doing business - there's an _enormous_ amount of very high value skills you can learn from them.
"A bunch of salespeople in suits tossing around buzzwords so they can land a job taking advantage of a big company's big budget. Everything is a pitch or a comp or a big lead."
That is indeed exactly the difference between most "agencies" and most small design firms/freelancers.
The agency has salespeople who know how to value and sell the work a freelancer can do, for an order of magnitude or so more money that that freelance will negotiate for themselves. You might think "I could give that company what they want in a few days with WordPress, a great theme I'm already very familiar with, and a solid day's worth of graphic design and css futzing - I'd do it for a friend for a few grand, but they're a big company so I'll see if I can get away with charging them $8 or $10 grand…". And you'd quite likely lose the job to the agency who comes in talking about Business Goals and Website Goals, Audience Demographics, Conversions, SMART metrics, Information Architecture, User Interface and User Experience design, Content Inventories, Conversion focused and SEO focused copywriting, Social Media integration, broader alignment with current marketing activity, leveraging existing business relationships and co-branding key messages - they'll spend two weeks (billing by the hour) talking to key stakeholders and decision makers at the client (while dressed, as you point out, in smart suits), then submit a proposal for a $280,000 project and, in case the budget doesn't stretch that far, a simpler $150,000 version. And they'll also have the known-effective "sales closer" tactics, probably something like "we've got a few slots open in out pipeline next quarter, I'm pretty sure if we could get this approved and signed before the end of the month I could talk finance into a 12% discount on a full upfront payment…"
The _good_ agencies will actually deliver a lot more business value that a freelancer with a good design eye, a folderful of WordPress themes, and a GoDaddy hosting reseller account.
A _bad_ agency will just have search/replaced the company name in their previous pitch powerpoint decks and web project proposal docs - and deliver a not-very-varefully-planned canned-theme WordPress site anyway, probably farmed out for $8k to some freelancer with the promise of heaps of future work and some great exposure…
Knowing which clients are going to get $100k+ value out of a project, then pitching a proposal based on value delivered, rather than hours worked. _That's_ what a successful agency does. (And what most freelancers have very little idea how to do.)