i think that even though the resemblance to the human heart is very vague, that's still the reason the stylised heart is as it is. things do morph over time, y'know, and with something like the heart symbol, with all it's romantic connotations, i don't think it strange that its form has gradually become something more symmetrical and easy on the eye.
as for stars, looking up at them in the sky, its a bright middle bit with "twinkly" bits coming off it, to the naked eye, so following the above logic, i don't think it's too odd that the symbol should have ended up as it is - i.e. symmetrical, stylised and easily reproduced.
either that, or it's all a conspiracy by handicraft fans, who can far more easily reproduce larger numbers of identical templates and motifs by folding and cutting paper is the star and heart symbols have more symmetry.......