Well, it is true that it is for security reasons. What if your devices are affected by malware etc.
I would not want any notes that may or may not be important ( well I am the type who Worry about security, have 3 Antivirus installed)
Below is from https://developer.android.com/guide/topics/data/data-storage.html#filesExternal
Internal Storage
Store private data on the device memory.
External Storage
Store public data on the shared external storage.
Using the Internal Storage
You can save files directly on the device's internal storage. By default, files saved to the internal storage are private to your application and other applications cannot access them (nor can the user). When the user uninstalls your application, these files are removed.
Using the External Storage
Every Android-compatible device supports a shared "external storage" that you can use to save files. This can be a removable storage media (such as an SD card) or an internal (non-removable) storage. Files saved to the external storage are world-readable and can be modified by the user when they enable USB mass storage to transfer files on a computer.
Caution: External storage can become unavailable if the user mounts the external storage on a computer or removes the media, and there's no security enforced upon files you save to the external storage. All applications can read and write files placed on the external storage and the user can remove them.