1. Journals have length limits. And readers don't read long papers at all.
2. Many academics have not worked hard enough to attain excellent writing skills - they were too busy becoming excellent at their discipline.
The most important reason.
- Given a clear lucid explanation of new knowledge that no one but you know, is incredibly difficult. Honestly, anyone, who thinks it's easy should take a stab at undertaking a research project and then trying to summarize months or years of work into 6-10 pages.