Because at most hotels, breakfast is mainly priced for those on expense accounts. They probably don't get a lot of calls asking how much the breakfast is: people know it's over a pretty high multiple over the "street price" at other options nearby, but the people buying it generally don't care.

The occasional exceptions I've seen are where a hotel is marketing a very high end option that they want you to make a reservation for. $50+ breakfasts that include multiple buffets, raw bar, Champagne, etc.

At a typical U.S. Marriott/Hilton, it's usually in the $20-25 range for the typical buffet. (Higher in select cities, of course.) Around 3x what I'd actually be willing to pay for it if I didn't have elite status...

For an international trip where I truly had no idea what a hotel would charge for breakfast, I'd probably just shoot them an email or a phone call.