Some of the answer is that these aren't really "different" devices. They're different cases produced for different carriers, etc... The actual number of true SoC platforms is actually fairly small.
But most of it is that hardware manufacturing of low-margin products is hard. Sure, you might really want to use a partcular Snapdragon part, but Qualcomm can't give them to you in the volumes you want, so you make two boards for two different chips. Or the camera manufacturer you were using just had a glitch and can't give you the second batch in time, but with a quick PCB rev you could use this other sensor which is "mostly" the same, etc...
Apple doesn't do this because Apple ships high margin parts and can pay more (i.e. order extras, pay premiums to get first priority in shipments, etc...) to get the components they want. And, frankly, because Apple is willing to eat the occasional supply burp and just spin it as "demand was too high!".
But most of it is that hardware manufacturing of low-margin products is hard. Sure, you might really want to use a partcular Snapdragon part, but Qualcomm can't give them to you in the volumes you want, so you make two boards for two different chips. Or the camera manufacturer you were using just had a glitch and can't give you the second batch in time, but with a quick PCB rev you could use this other sensor which is "mostly" the same, etc...
Apple doesn't do this because Apple ships high margin parts and can pay more (i.e. order extras, pay premiums to get first priority in shipments, etc...) to get the components they want. And, frankly, because Apple is willing to eat the occasional supply burp and just spin it as "demand was too high!".