One reason for software bloat is DRY principle. From very first steps every programmer told DRY! As a result modern software usually has many abstraction layers

One reason for software bloat is DRY principle. From very first steps every programmer told DRY! As a result modern software usually has many abstraction layers (because with many layers you can reuse more code) and hundred of dependencies (instead writing/copying a few lines of code we just add one more dependency, which has it's own dependencies and so on).