Just to prevent people from misunderstanding that: LÖVE does not stop you from going outside the read/write directories (The Lua IO library can). The love.filesystem module is designed to only access those 2 places for cross-platform reasons.
To clarify Boolsheet's point further, if you look in an external directory using the Lua IO functions, it won't be the same for each OS that LOVE supports. Directory structure is completely different in Windows, Mac, and Linux, adding external pathing will have to account for all three in a unique way and then things get messy.