Can you say what gives you this impression? Because I’ve been around for a while and have never heard anything like what you are describing (in fact I’ve never heard the word combination “endowed postdoc”, so although I get what you’re referring to, I think it’s best not to use this term).
If I had to guess, I think you’re conflating prestige with practical matters that are unrelated to prestige. Getting teaching experience is somewhat important for people who want to get a TT position at a good US (among others) university. But that has nothing to do with prestige, who the position is named after, or where the funding for it comes from. For example, an NSF postdoc is considered more prestigious than most of the postdoc positions you’re referring to as endowed, and, while it comes with reduced teaching responsibilities, is not mutually exclusive with having them. Other prestigious postdocs that don’t meet your definition of “endowed”, and don’t have teaching duties, are postdoctoral fellowships at places like MSRI and IAS. Anyone considering between such a position and another one that does involve teaching would need to carefully weigh their options based on many parameters, including prestige, the teaching experience they will be gaining, and various other things. In many cases it will not be an obvious choice based on a simple rule of the form “position X is always better than position Y”.