1. The design of documentation is usually atrocious (PHPs language docs make me throw up in my mouth a little every time I see them)
2. Documentation is written by super smart people designed the software... they tend to have a tough time relating to normal developers and especially beginners
3. Docs never provide enough examples. Stack overflow literally exists for this reason
4. Documentation is an after thought for most pieces of software (laravel is a notable example to the contrary - the docs are part of the software design for laravel), and not a forethought.
5. People just don't read period
6. Documentation usually has contrived examples, where other sources often have literally the think that you were trying to do (Users, Posts, Payments, Tags, etc)
Short list... but hopefully helpful