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One reason is that tablet phones keep getting gimped or eliminated by US carriers. They killed the phone functions of the original Galaxy Tab, likewise I can't get a Note 8 with the phone stuff built in.
Phones this size aren't for everyone but apparently a lot of people are spending money on them.
You may not see a need, that doesn't mean there isn't a demand.
2 years ago on Can Somebody Please Tell Me Why We Need Phones As Big As The Galaxy Mega?