[mcp111]

Then we can free up main phone memory

[hallux]

Because SD cards are significantly slower than internal storage and quite a bit less reliable..

[mustang7757]

I agree with hallux, you can still free up memory by putting pictures/video/documents on sd card but would also back those up to cloud just in case it gets corrupt.

[L0n3N1nja]

It causes severe performance issues because SD Cards are less than 1/10th the speed of UFS storage in modern phones, can't read and write simultaneously, and are less reliable.

Basically it creates a bad user experience.

[B. Diddy]

Which phone do you have? Some phones do allow the SD card to be formatted as Internal Storage (i.e., Adoptable Storage), but this brings up the issues that have been already mentioned. So certain manufacturers decided not to implement it.

If you need more storage, then upgrade to a phone that comes with more internal storage. Phones that have a small amount of internal storage are meant to be budget phones -- you get what you pay for.

[Mooncatt]

Which phone do you have? Some phones do allow the SD card to be formatted as Internal Storage (i.e., Adoptable Storage), but this brings up the issues that have been already mentioned.

There's also the issue that it permanently ties that card to that specific phone. You can't even plug it into a card reader and access it without first reformatting it and losing any data on it.