I am currently working on a GSM connected device and I feel like I have something to contribute, although I am no expert so be sure to do further research if this doesn't satisfy.
The GSM module I am working with states something along the lines of this in its datasheet (re-worded for NDA purposes, and to make it more general):
A SIM detection pin
must be used in order to comply with the 3GPP TS 11.11 document recommendation if the physical design of the mobile equipment allows the user to remove the SIM card during operation.
I combed the mentioned 3GPP document and did not find much talking about this so I am unsure of how well this is specified or if that point was from an older version of the document. Section 11.2.8 "SIM Presence Detection and Proactive Polling" of that document defines a polling procedure to detect SIM removal but only at critical times (what they call "card sessions", for example a phone call).
In any case, my theory is this: this is done as a design decision to simplify the phone firmware, as the phone would only ever need to do its SIM card initialization on power up, and to avoid having to comply with any possible procedures which only apply if you allow the user to remove the SIM card during operation. Seeing as there isn't a clear advantage of providing that feature, it's a no-brainer (for me at least) to go with a design that does not allow SIM hot-swapping.