It has to do with their infrastructure setup. When you visit another world, you are connecting to a different physical server machine which has no association with the other ones, and therefore can't necessarily access certain information. Ideally they should detangle the code for it and have this information able to be accessed across worlds like FC chat and CWLS.

Most likely the issue with mail is just the code. It was created in a time when we did not have world visit and will likely make certain assumptions about the world you are currently on.

Most likely the issue with timed nodes is you could farm them by visiting other worlds and it is hard to prevent that due to the fact you visit an entirely different physical server which is like a clean slate, and they don't want timed materials to be farmed.

Most likely the issue with retainers is the fact you can put items up for sale and you are putting them up for sale on your current world. So if you are on a different world, then it makes it complicated because it would have to send information to a different physical machine. This, unfortunately, affects your ability to access retainer inventory as a side effect.