How do pet relocation service providers justify their costs?

The first time I paid for a shipping company to help me.
The second time, we did it by ourselves.
The second time was way, way more stressful.
The costs were higher the first time as we had to buy a crate, which had to be specially made, and flew in December when the prices were higher.
We also had a lot of support through the process, including having the crate made to order, and different ticket quotes compared.
There is quite a lot of paperwork to do to fly dogs internationally, some of which is easy to do, other parts of which are dauntingly complex.
You have to make sure you are complying with both local and international laws, find a vet that is licensed to do health checks, make sure your crate is compliant, and figure out which flights the dog can fly on.
Not all planes have pressurised and heated holds and not all airports will take dogs as cargo.

Some of that cost will be having the dog or dogs fly as cargo. Depending on their size, that may be several thousand dollars in itself.
Sheba, my German shepherd, had a ticket that was about five times the price of my own. We joke that she had more leg room too, though the movie selection sucked.
If I had it to do again, considering that the first flight back was cancelled due to snow and we were scrambling to get her rebooked on the same day that we could fly, before her health certificate expired, I would have paid the extra cash to have someone help me.
We saved about US $1,300 doing it ourselves.
I reckon I lost about 5 years of my life to stress when our local, normal vet’s nurse suddenly announced, as we were stood in the JFK vet clinic trying to get her paperwork OK’d, that the our local vet hadn’t understood our request and that they shouldn’t have done her health check at all.
The lovely, kind, wonderful JFK vet then went into salvage mode and tried to help us find a new vet clinic to perform a new health check and reschedule the flight, again.
Ten minutes later, our local vet who had actually performed the health check called back and said the nurse was wrong, that it was fine, and that here were the details we needed.
So with two minutes to spare before the JFK clinic closed on a Friday, the day before we were due to fly for the rescheduled flight, we got her paperwork stamped and she was safe to fly.
Throughout the whole process, everyone was as helpful and as kind as can be. On the other hand, it was also more stressful than the rest of the international move.
From the number of people in the JFK office who were turned away in the 2–3 hours we were there for having the wrong paperwork, no paperwork, or had misunderstood some key detail that meant they had to start again, it’s something that needs careful navigation and is not easy.
So they earn their money. They are also a business that makes a profit off each transaction.
If you are used to international shipping, are really good at paperwork compliance, and can figure out the laws you require, you can avoid using a company.
A friend did and found it easy.
If your dog is unusually large, so needs a crate larger than commercially available, or if you find paperwork stressful, or if you are flying to or from somewhere unusual, you may be much better off using a company.
Also note a lot of airlines will only put two dogs on any flight, so you need to make sure both of yours get on the same one.
Relocation companies charge a lot because relocation is difficult, detail dependent, expensive, and stressful. They take that on so you don’t have to.
Then they charge you the big bucks for making your trouble into their trouble.
Good luck. I hope you and your dogs do well in your new homes!