There are undoubtedly multiple reasons. One reason is that there are domains where the hard problems are integration problems. If the point of your program is t

There are undoubtedly multiple reasons. One reason is that there are domains where the hard problems are integration problems. If the point of your program is to poke/sample some physical object, talk to another opaque chassis/address space, execute realtime tasks (which in practice often includes UI/UX concerns), etc., then extensive integration testing is absolutely vital. If you focus on some other testing discipline and try to de-emphasize integration, there is a considerable risk of fooling yourself about whether your program actually does what it says on the tin.