I think it's simpler for most people to just use a single word or 2 when talking about things. For many people who are dealing with what they call a "toxic" person/situation, explaining over and over to others or even just in their own emotional processing it is far less exhausting to use a single word adjective.
In English, the first person narrative is always implied unless specifically stated. This is what makes English as a second language so difficult. Most other languages have verb conjugation and pronouns that clarify the relationship between subject, speaker and audience. Englsh also relies very heavily on figurative language such as hyperbole and metaphor. So the term "toxic person" is a hyperbolic metaphor for "person who I percieve as dangerous to me because specific characteristics that I associate with negativity cause a significant reaction in my mental, emotional or physical state of being."