Because you need to earn a living for one thing. And you won't keep your job for long as another reason (see strategic cases below because there are exceptions of course).
Anyway along those lines would you suggest that a salesman for jet engines for Boeing not take an order and simply tell the client that a Rolls Royce engine is a better fit for the application? (And yes I know that there are strategic cases where it can build loyalty by doing this and/or if the stakes aren't to large you can try to be helpful.).
So in other words the salesman's job is to, at the expense of the survival of his own company, to simply help the client (on his employer's dime) make the best decision and as a result he will still be able to earn a living because everything will just work out? And of course his competitors are doing the same thing, right?