(Not a lawyer)

The license does have consideration.

You provide some code. The other entity (the project) provides attribution and other things depending on the specific license.

Promissory Estoppel only comes in to play if one side doesn't provide anything of value, and so the bar of consideration isn't met. But to my knowledge, licenses already meet the bar of consideration, so promissory estoppel doesn't matter.

Money isn't the only thing of value. The exchange of one good or service for another good or service is still consideration, no matter what. And "attribution" and "the promise to hold harmless" are of value.

In addition to that, "consideration" doesn't have to mean "money" — in fact, consideration usually won't be monetary on at least one side of the contract (e.g. if I offer you $5 to mow my lawn, the lawnmowing is consideration). If your license requires people to give you attribution, that could reasonably be viewed as consideration.

(IANAL and obviously this comes with the usual truck full of caveats that all legal issues do, but this is my understanding of the general state of US law at least.)

The consideration with open source licenses is often authorship credit / promotional. It's not terribly different than a band giving away MP3's of their music: maybe they want to share their music with the world, maybe they want you to buy a ticket to their concert, but they are getting something out of it and are free to give away something if they have the rights to do so.