@PTRB
It doesnt exactly fit nowadays but for those with wealth/power it can happen. Today clothes are much simpler so it doesn't fit as well currently, but if something is always done for you it can be easy to never bother paying much attention to it; especially if you don't know that it is anything unusual.
Anyway, its based on the fact that nobles/knights and the like would often have assistance when dressing or otherwise preparing. Some clothes/armor it was actually entirely necessary for assistance in getting dressed as they were difficult, if not impossible, to do yourself. Check out what it sometimes took to fully equip a knight for ceremonies or battle. The nobles or others clothing may not have been quite so difficult but it was far from the simplicity of most modern clothes.
The simple example that modern people can more easily understand would be dresses that have a zipper which is unreachable by the one wearing it. Many higher class outfits would have even more complexity- if you gave an era accurate outfit to a modern person they may struggle with even just getting the proper order that you would should wear them. Another quick example-If you know enough about yukatas you know that just handing them to someone without experience often doesn't look right, if it even stays on properly and again that is generally less complex than the full outfits an ojousama may be required to wear.
Since modern clothes generally don't require as much work this type of service isn't needed as much, but it is used. In the case of a high class child if they never need to dress themselves they may not learn even the basics. Especially if they lack a 'normal' social life, from their perspective it is entirely normal to simply be dressed by another and not worry about the details. Similar idea to not learning about cooking unless otherwise needed. Heck look at the many people now who basically can't cook a thing- from family/friends and stuff like prepared food or fast food; why go out of the way to learn about it if you have no reason to- especially as a child or young adult who has never had exposure to other trigger it. Not saying I agree, just explaining some of the reasoning behind it.
TLDR- It is a bit exaggerated in many stories now but it does have a basis in reality; more than a lot of other common troupes anyway.
Edit- Saw it after posting, sorry for the wall of text guys lol
I thought that would be shorter and I left plenty out.... Anyway had time to wait while chapter loaded and thought it could be...interesting (not really useful to know for most people) to provide some background if it wasn't known.