"Pay later" is a benefit, to the audience we're discussing, as it means "not now".
Young adults actually have processing problems over impulse control [0], and so cannot see any drawback to "not now" - in fact they likely do not have a way to fully conceptualise that something will occur in the future.
All "buy now, pay later" translates to is "instant gratification", nothing further.
[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2964271/