There's no zone information in the capture timestamp. Why?
CIPA DC- 008 is the standard for Exif 2.2. Of note it makes no mention of "timezone." "GMT" is also not mentioned either. The term "UTC" does appear but only specifies the GPS time is recorded as such. Pedants may note that this is slightly misleading since GPS time is not identical to UTC
My point, in short, is that omission of timezone is likely not accidental. I'm not saying that CIPA or JEITA thought it was a good idea to leave timezone out. I have no way of knowing how they felt about the decision. The fact that UTC is specified in one time tag and not in another means that the creators of the standard were aware they were leaving it out.
I can't say for sure but I am willing to speculate that usually when something is ambiguous in a standard it is because of a disagreement in the committee that created it. I suspect that unless somebody unearths the transcript of the discussions around creation of EXIF we will never really know why.
EXIF v2.31 (p49) defined time-zone offset fields in 2016 and the XMP time-zone guidelines (p33-34) also consider time-zones. The implementation in cameras and programs is rather minimalist at the moment.
See also my answer to an older question: What do you do with your camera clock time in relation to time zones?
Look at a Time Zone map. Time Zones are local political decisions, not necessarily agreeing with simple 15 degree longitude lines. For GPS to get it correct, the camera would have to have extensive lists including everywhere time zone did not map to longitude. Our computers and our cameras can't know this, and don't try to deal with that, they simply ask which time zone we want to show.
Knowing GMT is a lot to know, but we can set our camera to show whatever time zone we wish.