Because that is already promised!?!
Including no break of warranty, if you manage to flash back to Android.

a. Yes, quite closely.
BTW, the Librem, PinePhone etc. use the same design principle: No shared memory and no Bus Mastering from a separate modem.

b. If the there is no shared memory and no Bus Mastering from a separate modem, a "kill switch" makes basically no difference.

c. When an integrated modem is used, a true "hardware kill switch" is impossible.
But a separate modem needs extra power, space, costs etc.

d. With a Qualcomm SoC as in the Pro1 an (technically excellent) integrated modem comes for free, while NXP's (Librem) and Allwinner's (PinePhone) SoCs lack one, because they are designed for embedded control applications; hence they have to use a separate modem anyway.

e. I doubt that a true "hardware kill switch" will work fine even for a separate modem, so I am really curious to see how these switches are implemented in the final schematics (of e.g. Librem and PinePhone) and / or to hear how well they works in practice.

But yes, I am long convinced that there is a small but very vocal group of believers, who direly want something called "hardware kill switch", even if it makes the device bigger, heavier, more expensive, running hotter, reduces battery runtime etc.
Still I do not believe that this group will generate significant sales, especially as the niche of devices with "hardware kill switches" is already occupied by two at the moment.
But a separate modem, when technically unneeded, would reduce the appeal of a device for the average user.