A package means that it can be written as a "package", but it doesn't have to be written as a "package". There is such a linguistic phenomenon that people tend to regard some words appearing in books as authorities. This situation has caused a word to be regarded as the only correct word once it appears many times in the book. This psychological habit causes some words with the same pronunciation and meaning to be considered wrong. However, in fact, there are many words with the same pronunciation and meaning in Chinese. On the other hand, there is "immortality" in the dictionary, but there is no "immortality" because the usage of "treasure" in "Immortality" is quite special.

As a language habit, it may not be logical enough. Just like mom, why add a horse next to a woman instead of a cow?

I believe most people don't know the answer to your question. As a conventional communication tool, language needs you to adapt, just like everyone is using "keep young forever", it is just a tool for you to express to most people. Of course, you can also habitually use "eternal youth", but because of people's language habits, many times you will be forced to make excuses, unless you think it is worth it, you will be very tired.