not surprising since Android was born from Linux.
https://www.reddit.com/r/Android/comments/3o6u80/psa_i_formatted_my_sd_card_as_internal_storage/
Before KK, any app could access any and all the data on extSD(here on just SD). Obviously this was a giant security hole. So they upped the sec. game with KK and restricted apps to their own folders on SD. This broke functionality of a lot of apps, like gallery/camera apps and several others. So in LP, Google offered a median solution, letting apps request access to whole tree, getting access through file/doc picker via API. Even this wasn't enough for some though, as it lacks any "seemless" feel and is quite janky.
\You'd ask, why all this? Simple. Almost every SD card out there is formatted with FAT file system. It is a proprietary FS from Microsoft. Also, it lacks some very imp. features, like per file permissions. This was major reason behind pushing SD cards away from central stream.
With MM, Google finally came up with an elegant solution. Now, they simply format SD card to EXT4, same FS as your internal partition(on some phones now, its F2FS, but for standard AOSP, its still EXT). This way, Android™ can have all the FS features on SD, all the control including permissions and encryption(optional). This also makes SD cards just another partition on the phone to be used for data.
Drawbacks are initial formatting of data on SD cards, and SD card removal making everything on it, inaccessible. But given benefits, its the best approach Google could have taken. I'm surprised it took them this long, given how many droids are sold with meager internal storage and SD cards as expansion.
TL;DR: SD cards had a few issues, Google tried bypassing, messed up, finally fixed the OG problem. SD cards = good now.