What would anybody even use 4 TB SD card for? Storing a shit-ton of pirated movies that you can watch on your phone? Aside from that I have no idea. 256 gigs is probably more than enough for anything a normal user would do on a phone.
Portable gaming Pcs. I would love to have my entire library of games accessible offline. My emulation folder alone is like 500gb. I also wouldnt call myself a normal user though. These definitely have a niche market and probably a price tag just as niche.
File size is a major limiting factor in high speed video and to a lesser extent convenient ultra HD digital film.
At 3840x2160 (basic 4k) uncompressed 10-bit video 1 frame is about 250 MB. An hour of footage at 30 fps then is about half a terabyte. At "only" 1000 fps you would burn through an 8 TB SD card in... 32 seconds.