it’s not that free labor isn’t worth it, it’s that it’s a lemons problem. that is, (the signaling indicates) uncertainty is high, which is what makes it expensive (or if lucky, a fantastic deal). most people are rather risk-averse and therefore prefer safe choices, with the commensurate relinquishment of potential greater gains (and greater losses).

the lesson should be to value your labor correctly, as large deviations in perceptions of value mean that transactions won’t happen. a more astute approach would be to communicate your (accurate) understanding of your own value and your willingness to negotiate alternate dimensions of value in exchange, like getting experience faster or doing more interesting work in exchange for less money.

tl;dr: establish a common understanding of value, then negotiate.