This ones for id4me. What you're noticing seems to be the basic element of profit. It's why we go to china to manufacture. The bigger the difference between what goes out and what comes in gives you profit in a simplified way.
-Companies hire design firms for 10k-500k for a product design. The product will make waaaaay more than that if it's successful in the market. The company makes profit off of the design firm that way (extremely simplified but you get the picture).
-Design firms hire designers to design and charge clients for the designs. The bigger the difference, the bigger the profit. So you pay your designers the least that you can for given conditions (market, competition, etc) while charging your clients the most that you can for what you offer. That's how the firm makes profit. They have to pay out less than what they get to charge for the work, they're a business.
If you want a bigger share of the pie for yourself, you have to be higher in the chain by going as close to the "selling to the masses" phase as you can. That may mean freelancing or consulting directly for a big corporation (bypassing a consultancy that'll pay you much less than they're billing, even if you would be the only one doing the work), or even higher up would be to become a corporation yourself that makes/ directly sells your own products. The closer to the customer you are on the chain, the more potential (and risk!) you have for the big bucks.
Granted this is an extremely simplified version, I think you get the picture. If you have an idea for a great product you can market your idea / skills to the 10 local firms for $, the 20 global manufactures for $$$ or (with extreme risk) make it yourself and have the billions of people in the world to potentially market to for $$$$$$$ (what corporations typically do).
These aspects are why you generally don't get the best pay as a regular "in the trenches" designer in a firm.
I see I've run off on a slight tangent so to go back on track, I just don't think there's anything else a design firm can do short of being well managed and making the work environment as pleasant as possible for their designers. This can be done by having clear goals and expectations, trusting your design talent (no micromanaging) and giving your people the tools they need (space, resources, equipment, respect, programs, etc) to get their job done with the least amount of drama. Most of us are in the field partially because it's kind of fun and not drone work like a McJob. So if the environment is fun, work doesn't seem like work drudgery and being able to enjoy every day and not wake up hating that you have to go to work makes up for the lower pay.
I don't think the pay issue can be overcome unless the firms have other revenue sources such as their own products in production, being on retainer for a company (consistent money), having very low overhead possibly by being virtual or through other tactics, or they're already so successful that they constantly have positive moneyflow to the point where they have to turn down work / don't have to ever worry about it. If those conditions exist, they can probably pay their talent more. If not and they're basically hunting for the next big project to make sure they can pay everyone and expenses, you'll find the pay for the designers will stay low since that will probably be their biggest overhead.
With all of the competition out there it's probably like that for a lot of places. So I'd say that they should just make sure the work environment is as good as it possibly can be for the designers and give them the tools and respect they need to create. Don't skimp on your designers tools and mental / emotional health, that's the foundation of your firms design output.