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.
Even after you do, you still have to find a definition
LOL! 99% of people absolutely do not look up a new word when they hear it. If the listener thinks it makes the speaker sound smart then they get a vague idea of the meaning from context and then start using it – often in the wrong context. All the dummies of the world repeat this process and it spreads like a virus.
And thus, another word with a very specific meaning gets turned into another broad-meaning synonym. If don’t believe me I’ll caveat all over your nuances until you verbiage.
Much like you and the comment you replied to, people DO "find a definition" without looking it up. They use context to assign a definition. They may end up inferring a completely different definition than anything you've heard, but they have defined it in some way.
A symbiotic relationship can be many things. Mutualistic (both organisns benefit), parasitic (one benefits, one harmed), and commensalism (one benefits, one is unaffected)
I was taught that symbiotic relationships are mutually beneficial, and is the opposite of a parasitic relationship where one benefits and the other suffers. I've never heard it used in any other way.
It's also how dictionaries define "symbiotic."
It seems like one of those things that only gets better defined if you're in the field of study, such as a biologist.
This comes from a memory of a digression during a lecture in an ecology class I was in 20yrs ago… so you know, grain of salt.
From this particular professors point of view. Symbiotic was the term to describe mutualism until recently. And then. A few papers started using symbiosis as an umbrella term for all relationships with sub-terms to describe the “benefits math”. This, to him, was annoying pedantry. But eventually all the textbooks adopted the new hierarchy of terms and the world moved on.
If you took a biology class with a text published pre-2000s, it’s very possible that your book described symbiosis as a mutually beneficial relationship between species.
Long story short: the language is fluid and ever changing, even in science fields.
Because they don't know the difference. People don't tend to use expressions or sayings wrong on purpose. Most never investigate the meanings or etymology, they're using them and are understood by the like-minded.