There are just too many options in each drawable class, to replace them with a single drawing function. There would be too many arguments, and you would probably have to set all of them, even the ones you don't care about.
Using drawing functions also means that you must store the drawing parameters on your side, so using classes helps you write less code by aggregating these parameters into single objects.
And of course if you really like drawing with functions rather than objects, you can easily write the corresponding functions yourself, with only the arguments that are relevant in your context.
void drawSomeStuff(sf::RenderTarget& target, arg1, arg2, arg3, ...)
{
sf::SomeDrawable drawable;
drawable.setStuff1(arg1);
drawable.setStuff2(arg2);
drawable.setStuff3(arg3);
...
target.draw(drawable);
}