It seems likely they only appear that way because the initial cost is amortized into the contract for most people in the US. An iPhone 7 is around $700! And flagship android phones are up there too. I guess people have a lot of disposable income or just see the sign-up price?
Starting price for the IBM PC was over $4000 in current dollars, for a dramatically less-capable device. It's not that upgradability wouldn't be nice, it just isn't as important as portability, reliability, battery life, or initial price.
From what I've seem most people end up dropping and smashing them every 5 months, they're well aware of how much they cost because they'll happily slide their fingers across shards of glass to avoid paying again.
One could say that's a good reason to push the modular phone.