Because the purchase is complicated and it costs you nothing extra. There are dozens of ships, a dozen or more operators, and itineraries that look similar on paper but deliver quite different trips. Our job is to cut that down to the two or three that actually fit what you want, and to be straight with you about what each one gives up.
The trade-offs are where the value sits. Almost every choice on a polar trip costs you something else. Flying saves you the Drake and rules out South Georgia. A smaller ship gives you longer ashore and a rougher crossing. Peak season gives you chicks and the highest prices of the year. Knowing which compromise you'll actually mind is the part that's hard to do from a website.
Support also runs past the booking. Packing lists, tipping, gear, what to do with the two days you have in Ushuaia. Operators hand that back to you once you've paid.
If you're weighing up agents generally, have a conversation with them first and see how it feels. Ask around the polar communities. This is a small and fairly cooperative industry and reputations are visible.