It turns out it's worth it to merchants because when you're not paying now, you end up buying more than you would otherwise. Order sizes are ~15% higher [2]. Probably similar to how it hurts more to pay with cash than debit because it's so tangible.
I view it kinda similar to gambling apps with their endlessly optimized special offers designed to exploit the human monkey brain.
[1] https://www.fool.com/investing/how-to-invest/stocks/how-does... [2] https://www.uschamber.com/co/good-company/the-leap/klarna-se...