you say that but having agents sell your product, advertise your product and then deal with the mess should something happen boosts your profits.
instead of only selling to say 1million direct customer... you might sell to 900k direct customer but then to 4million customers through agents that you use.
By selling your product through an agent you open your product up to every shop, call centre and online agent out there which will use their own budgets for marketing ( some of which yes it partially paid for by by certain suppliers )
But as an independent agent I will switch sell a customer to the best product for them... if that happens to then be your product then thats a win for you as you have paid nothing to get the customer to book with you other than the commission and its revenue to you that you would not have got otherwise.
This is what suppliers like Riviera Travel and Saga had to do. Not that long ago both these companies were direct sell companies only, however it was costing them so much to bring in new customers to the business as they were having to bid against other companies advertisements they could not keep up with the marketing budget.
Now if we advertise a Viking cruise... and a customer calls up about this and its too expensive for them il switch sell them to Riviera instead which is a cheaper alternative but still very good. That a booking that Riviera have then got without the need to pay for the marketing to get it.