Earlier quoted context omitted.
Are phone SD cards encrypted? Because they're not fast or reliable, so one would at least hope they're secure.
The newer microSD cards are quite fast. Not as fast as built-in storage but fast enough for things like photos and videos if you buy a card with the appropriate speed class. V30 512 GB costs about 40€ including VAT here in Germany. https://en.wikipedia.org/wiki/SD_card#Class
I use a modern high-end SD card from SanDisk as semi-permanent storage in my MBP. If I copy anything to that card that's not a single file, it takes forever. Overall it feels worse than using an old HDD. I have to actually think about what I store on it (mostly videos/photos), so I totally see the reason for wanting more internal storage instead of an SD slot.