Why loaners only provide preapproval but not prequalification letters?

[Leo Sun]

I plan to start my search of houses this quarter and planned to get a prequalification letter first. With it I can have a better idea of the price range I can afford.

I contacted BOA and Chase loaners. And they all told me that they are only willing to provide preapproval letter, but not prequalification letter.

Can you help share some insights why bank loaners do not want to provide prequalification letters?

I do not want to get a preapproval letter now as I can not afford the credit score decrease if I can not find the right house in 90 days. My current credit score is around 750. If I do a hard pull but can not buy a house in 90 days, I am worried that I can not get the best interest rate after a 20-50 credit decrease.

Any suggestions how I can get a prequalification letter? 

Thanks!