Based on what I know, a large portion of the loans have very small amount and short duration. For instance, people want to borrow 1000 CNY for a month and pay back 1020 afterwards. That's already 2% per month.
EDIT: The reasons they need such loans are more complicated. Some needs money for emergency. Some uses the money to buy stock/cryptocurrencies...
EDIT2: P2P platforms typically take money from the deep pockets and subdivide into tiny loans. They earn the interest rate difference (e.g. 20% from users vs 15% to capital providers).
> high defaults
Empirically the default rate is very low. People pay back on time if you call or just text them. Honestly if they don't pay, it's totally fine, and the P2P platforms just don't care, because they earn so much money...