Not to defend any particular builders practices, but depending on state, cost of job, local council, etc the builder is taking on some risk. They have to offer warranties and accept various liability depending on the local rules and regulations. Their insurance isn't cheap and looking at a structure where the roof is supposed to come off is probably confusing for many and looks just like legal trouble when it blows off in the wind.
You could get an engineer to produce a design that would involve them supplying drawings that any builder should be comfortable quoting off, but that would cost a lot probably.
As an example take a look at the details of being an owner-builder in Vic:
https://www.vba.vic.gov.au/owner-bui...-owner-builder
There's a lot of rules and regulations and as you can see they depend in many cases on how much you are spending on the work. (not suggesting you should be an owner-builder or that this info is even related, its just an example of regulations that may affect a builder's quote, or willingness to provide one).