If you look closely in the night sky they sparkle and if you move your head a star will seem to have tiny moving points. Planets tend to have a hard dot shape. The sparkling of stars is probably due to a combination of the atmosphere and the imperfections of your eyes trying to focus on something that both incredible small, and incredibly bright. Imagine staring at a flair as bright as the sun from a mile away.
On top of that cultural conventions come in. Human hearts dont really look "heart shaped".
I admit that putting points on stars may not be a universal convention in every culture.
Centuries ago When westerners in sailing ships began to trade with the xenophobic Chinese the Chinese referred to all Westerners as "foreign devils". Later when Americans began to sail to China this new group of westerners were referred to as "the flowery flag devils" ( to distinguish Americans from Brits, Portugese, etc) because they thought the stars on our flag were supposed to be flowers.