Requested pre-approval for a land/lot loan. Why does lender withhold letter until they get property info?

We gave our credit union all of our credit information (w-2's, pay stubs, bank statements, tax returns, etc.). They've said by telephone they would approve us for even more than the amount we're interested in borrowing, but they're asking us to begin negotiations with a realtor about a property... show more We gave our credit union all of our credit information (w-2's, pay stubs, bank statements, tax returns, etc.). They've said by telephone they would approve us for even more than the amount we're interested in borrowing, but they're asking us to begin negotiations with a realtor about a property we're interested in before they will give us a pre-approval letter to take to the real estate agent! (We haven't started to work with a realtor yet). We feel like it's a Catch-22, because usually a realtor wants a pre-approval letter before negotiations even begin. Why would they need specific property info for a pre-approval letter which is not binding on them anyway?
Update: In fact, there is such a thing as a mortgage for a land/lot loan. Interest rate is 7.0% for a mortgage on the unimproved lot. We HAVE filled out a complete application. We were told it would work just like a preapproval letter for a house, so if we wanted to keep looking, we did not have to reapply for a loan on a... show more In fact, there is such a thing as a mortgage for a land/lot loan. Interest rate is 7.0% for a mortgage on the unimproved lot. We HAVE filled out a complete application. We were told it would work just like a preapproval letter for a house, so if we wanted to keep looking, we did not have to reapply for a loan on a different property if we found one.
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