Talk to admissions officers or check some of the guidebooks about college admissions and you will find that yes, indeed, it is rare that trips abroad to volunteer impress admissions officers. They are savvy enough to recognize that volunteering abroad typically means: a student's parents have buckets of money and are willing to use that money to buy an experience that the parents think will be impressive; the students have relatives abroad who sign off on some "volunteer" work that really is a sham; the student participated along with a group of other American teens in a very structured, chaperoned program in which they did some simple tasks that wouldn't seem impressive if done in the U.S.
The admissions officers also know that an inexperienced American teen isn't likely to have any kind of major impact by volunteer for a couple of weeks abroad. The same teen, however, could have a major impact and learn a great deal, too, by volunteering over a period of years in their own community or by organizing a service project in their community. Even a modest service project requires far more leadership, creativity and organizational skills than do packaged volunteer trips abroad.
So, you are right: In most cases, admissions officers will be far more impressed by a student who has done longterm volunteer work in the U.S. than by a student who has done some kind of fancy sounding volunteer experience abroad.