OK, that heading is probably not true if you drive around in a brand new luxury limo. Some of those kind of cars have a twenty speaker or more and a powerful separate amp.
But the majority of medium to cheap runabouts are fitted with sound systems that are about as cheap as the production car plant can find.
Also, over time speakers will lose clarity and aftermarket replacements will make a huge difference to the quality of sound. Even sticking with the cheaper coaxial speakers will be an improvement.
Any improvement to a car sound system will always involve some anxiety about cost. Like so many things in life, you get what you pay for and the bigger your budget the better.
If you really want a huge improvement then look at the exercise in new component parts spread over a few monthly paydays.
You may want to start with the new car speakers installation and continue for a month or so with the creaking old header unit. The next swap could be the header unit and finally at least one new sub-woofer.
The best quality component speakers will get you on your way to a stunning new custom built musical experience.
One advantage that more expensive premium speakers have is the quality of the material surrounding them. The foam and paper of cheaper models will deteriorate faster than the far more solid ones.
Just replacing the factory speakers won’t mean much if the low powered head and amp are not also replaced by better quality.
Two way and three way speakers give a much crisper sound than the full range ones that are probably the type fitted in your vehicle now. They may cost a bit but the improvement in sound quality will more than make up for that.
To exceed even that quality, you need to be not looking at two or three way but dedicated woofers and tweeters. But if you choose this route then you’ll almost certainly need some professional help.