Because they talk about their products and services from the wrong perspective.
Inexperienced ‘sales’ people have an unfortunate tendency to talk from ‘inside to out’, rather than from the customer’s perspective in. Often the pitcher will start by describing their product and how it works before they get on to what it will do for the customer, if they ever do.
Clients are self-interested. They will pay you handsomely if you return value to them in some other way. Think ‘Dragon’s Den’ for a moment- The product being pitched to potential investors can be chilli sauce or waste disposal trucks, and the only thing that the moneyed ones are thinking as each pitch begins is “Will I be able to make money here. How much and by when?”. The details (so long as the business seems ethical and legal) of how the investment will be turned into even more cash for the rich people, is often irrelevant.
If the answer to the ‘Will I make pots of cash,? How much? and by when?’ is ‘yes’, ‘lots’ and ‘very soon’, then even a buffoon is going to have to work hard to lose the investment. But if the answer’s ‘no’, ‘never’ and ‘not in a million years’, you might as well be Al Pacino with Billy Shakespeare as your scriptwriter and you’re going home with less than you had when you walked up those post-modern, industrial stairs.
Clients have things that they want to achieve in their businesses and if you can show, clearly, how spending cash with you will make it more likely that they’ll win, then you’ve started well.