This isn't generally true.
Large and well-established open source projects usually have dedicated schemes to match new contributors with simple tasks.
For example, here is the GNOME program which gets you started with some hand-picked, newcomer-friendly tasks:
https://wiki.gnome.org/Newcomers/SolveProject
Similarly, LibreOffice has the "Easy Hacks" project:
https://wiki.documentfoundation.org/Development/EasyHacks