There two major reasons this matters:
1. In many cases no amount of enthusiasm for your business is more valuable than the hard skills you need. If it were, why can't you, the founder, with enough enthusiasm to start the business in the first place, simply do everything?
2. Genuine enthusiasm is *volatile*. If someone who isn't making business decisions cares deeply about the business' stated goals, they may well take a minor pivot or a strategic choice they disagree with as a slight or even a betrayal, and that enthusiasm quickly disappears or goes negative. In contrast, your indifferent professional has transparent and predictable motivations and you can stay aligned merely by continuing to pay them enough.
Personally, I think people select for enthusiasm about the business for the same reason they might select friends who tell them how cool or attractive or smart they are, and it's a not an amazing strategy for similar reasons.
I've seen people motivate this by saying that not faking it shows you lack social skills. In the end the customer is never wrong, and when you are selling your time the customer is your manager so keep that smile up! (I hate this culture, but I adapted just like everyone else)
It's impossible, unless he holds a large portion of the shares of the company. One of the goals of the company is to replace the guy with someone cheaper or find a way to no need him at all.
As a secondary effect, applicants of course notice that they are more likely to be hired if they just pretend like they care. And if everyone pretends, the one person who says they are just there for the money is at a disadvantage to everyone else.