Different people different financial management. A lot of reasons why one don't take 0% installment.

1. They think got hidden agenda.
2. They have the mindset that "installment means can't afford".
3. They prefer one-off payment so no need to worry about monthly installments later.
4. They lack of self-discipline - may use the money to spend on other stuff instead of paying monthly installments.
5. They don't want the installment amount to fill up their credit limit.
6. They may want to apply loan in the near future so they want a better DSR.

and so on.

I don't have that mindset. But apparently a lot of /ktards think that it's OK to take 3% installment to buy car but it's not OK to take 0% installment to buy phone. Maybe they can answer you.