Well, it is - because they had to make it with almost perfect backwards compatibility for one of the most popular languages with trillions of lines of code produced over decades.
Sure, adding it to a new language is not hard. Adding it to Java which has primitives, generics and boxing, finding ways that seamlessly cover the differences between objects and primitives, while trying to plan for the future is hard.
As a general note, if you come to the conclusion that one of the best designer teams on Earth "basically copied what .NET did from year 1 is not a good look", then maybe your mental model needs adjusting on how these stuff works? Java has a public mailing list, you can browse through the related discussions. Implementation is the least of these things. But I can assure you they most definitely know what they are doing.
At what cost? A key benefit of value types is improved performance but AFAIK Valhalla doesn't even let you pass them by reference. Efficiently passing them through registers is great but won't help you out with larger value types.
That's materially distinct from Java's model of basically dynamic loading already compiled class files. Though class files do have "editions", and there are extra code to deal with different versions. But still, it should be possible to e.g. send a new value class to an old library's class that has never heard of them, and that should just work.
Oversimplifying a big semantic and backend change to a huge codebase on which some of the most crucial customer and government and business systems depend on, and which has to be made as seamless, correct, and performant as possible, to "they just copied .NET", just because .NET has the same functionality, is an even worse look.
It's a "HN "Dropbox is just rsync + some scripts"-style bad look.