Because it's easier for the general public to understand. Yes, what you actually have is plasma - superheated ionised gas - but for public consumption, then you have to explain what plasma is and how it's nothing to do with the plasma in blood.
Try explaining what "radiation" is to people who are scared of microwave ovens. "No, there is no radioactivity, just microwave radio - radiation means lots of different things and only some of them are dangerous. Light is radiation too."
There are a few books on "The Science of Discworld" in which the scientists who wrote every second chapter explain education as "lies to children". Tell them something they CAN get their heads around, then if they go further, now we can expand that closer to reality. I well remember this from school - in England we had O level exams at 16 (now it's GCSEs) and studying chemistry would leave you with the impression that atoms consist of a nucleus with protons and neutrons in it, with much smaller little balls called electrons orbiting that. Fine as far as it goes, then I went on to A level chemistry and it was "ah, that's not the whole story, electrons aren't little balls, they're more fuzzy things in orbitals". Yep, at O level, I'd been fed a convenient "lie to children", all with the best intentions.
"Stars are balls of gas" is a convenient "lie to adults". Not the entire truth, but not a total lie either and it'll do for people who are just interested and don't want to go deeper into physics.