More than likely, actual back-end development got started well before you got to this point. The back-end developers had to set up the environments & platforms, pull in dependencies, figure out what systems would be used, plan the database, figure out what systems need to be integrated, and much more. All of this work takes time. Unless they have built the same or a very similar system in the past, they have days of work and research to do in order to get ready to do their primary jobs. Some of these tasks won’t be done until the team gets to the point where they are ready to check off that particular item on the list, but much of the work can be done ahead of time -- and the more that is done up front, the more accurate your time and financial estimates will be.