Ask Lemmygrad

There are two main problems.

First, global languages as a whole have barely anything similar to each other. For example, there are languages there isn't any way to tell relative direction (so I would call one of my hands my "west hand" and the other the "east hand", but if I turned 90 deg I would call them "north" hand and "south" hand). This is just scratching the surface.

If you had to list common features present in all languages globally, it would probably go like "all languages have vowels", "all languages have nouns", ~~"all languages have pronouns"~~, "most languages use m sounds to refer to mothers and d/p/f sounds to refer to fathers" etc. Like, the bare minimum. You can't even be sure if all languages have recursion (see Piñaha, although in my opinion the claim that this language doesn't have recursion is bullshit). There is so much variation that any "composite" language speakers would necessarily take some choices. If you had to take the features of the most spoken languages of the world, then the result would sound like a tonal European language. European languages are the most widely spoken because of colonialism.

Second, languages carry culture. Change a language, you lose poetry, puns, traditional history etc. Change a language enough and you end up killing it.

Why is "to shit" a slur and "to defecate" is not? Because poor Anglo peasants spoke a Germanic ancestor of English (where the word "shit" come from), and the aristocrat Saxons spoke an ancestor of French (where the word "defecate" come from). Why care about Anglos and Saxons, and their respective social positions? Because the Saxons invaded England and your "is this word profane?" sense came in part from their social status. (This paragraph might not be accurate. See the comment from La Dame d'Azur)

Also, if you want to see a "composite" language, you are reading one right now. English took features from all over the world due to colonialism and it's a mess. However, it is still relatively stable.