Because they're effective synonyms in common usage. All kinds of jargon get used outside of the original field.

In general, we all learn words piecemeal. You have to encounter a word to know it exists. The more specific and/or niche a word is, the less likely you are to run into it. Even after you do, you still have to find a definition. If that definition is simplified, or doesn't come with links to more information than a solid definition, that's what the person knows, and they can't know any other usage until and unless they encounter that too.

Now, that ignores the raw fact that language shifts as long as it is being spoken. Dictionaries follow language changes, and aren't really good at preventing shifts because they only contain partial information.

Symbiosis isn't going to have a full explanation of everything it entails in a general dictionary (though it might in field specific ones the way things like medical terminology have). It'll have a basic definition and some variants. If you want explant, you go to encyclopedias for basics, then to field specific texts/instructions if you want more depth.

As you anyway already said in a fairly compact comment, symbiosis contains within its definition other words. And you even gave a simplified definition of those. Now, anyone finding those words through this post will know that there are multiple "types" of symbiosis. But it is never the default to know things.

Ignorance is the default. We're born ignorant of almost everything. We die less ignorant than we started, though exactly how much less varies.

That's the reason people use the word in the colloquial sense; that's all they've encountered. As long as you don't act like a dick about it, most people will appreciate the kind of simple expansion you gave in your comments, and you can help people expand their knowledge. But you gotta remember that your pet peeves are meaningless to anyone else, just like theirs are to you. Come at it from friendly, kind frame of mind, and it'll work out best.