Why manufacturers glue parts (on decorated models)? Because they don't expect the models will ever need to be taken apart.
Why this is done for undecorated models? That is a good question. I can think of couple possible reasons.
1. The reason for selling undercoated models is not clearly conveyed to the overseas factory producing those models, so they just follow their standard assembly process. Most of those factory workers or management likely have no clue what those bare gray plastic models will be used for. Language barrier could be part of this problem.
2. The overseas factory simply refuses to modify their assembly processes, so they do what they want: see point 1.
The weirdest example of cluelessness at the overseas factory was some undec. Life-Like locos I bough some years ago. Yes, they were "undecorated", but those gray shells were not bare plastic. The gray plastic was actually "decorated" with gray primer!! So much for keeping my custom paint thickness to minimum. I had to strip the primer to get the shell into its virgin state!
Part of the problem is also the way the models are designed; They are designed to be assembled with glue. Kato designers are skilled enough to design their models with perfectly fitting parts for glue-less assembly. While their models might not be the most detailed, the ease of assembly and disassembly is most welcome for me. Kato has it figured out!
