why are soaps rectangular in shape

Because it's the easiest shape to make. Imagine if you have a blob of stuff that you have to make then cut into commercial shapes, what's the easiest cut to do? Aside from that rectangular shaped products are also more easily stored, because they can be easily stacked. That's the same reason why Japan developed their rectangular shaped watermelons :)

Hi, I've seen soaps in round, oval and heart shapes. The usual types are rectangular; I suppose easier to produce, as the manufacturer can do this in bulk. The others are sold in specialty shops, and cost more. But they are cute, and are good as gifts.

hello there... usually, soap which in rectangular shape are easy to hold and doesn't slip out of your hand... if you design the shop with irregular shapes, it will be uncomfortable to hold it... that's my thought...