Nepal is an outlier when it comes to flag shape, as it stands out from the norm (rather “two triangles stacked on top of each other”) since according to them: the flag represents Himalaya Mountains & both religions: Hinduism & Buddhism, also the red triangular flag has been a Hindu symbol of victory since the writing of Ramayana and Mahabharata.
That is in contrast to most countries flags as a majority are rectangular in their shape (no matter where from USA, UK, Canada, Japan, Germany, Poland, etc.). I mean, why are most flags rectangle by default rather than being unique on using another shape? I mean, Nepal is the only country where a flag is designed from another shape that differs from a rectangle.
On a similar note, I’ve often wondered why almost all coins are round
Many old coins were stamped from blobs of metal; harder to create a perfect square than a mostly perfect circle.
In india we used to have a paise which was a squircle
Round coins are less likely to damage coin purses and pockets. No more complicated than that.
Then as technology has moved on, it has proven advantageous for coins to have constant radius so that coin-accepting machines don’t jam on an unfortunate rotation of points.
This is why even when coins aren’t round - thinking specifically of the British 50 pence and 20 pence coins, but there are others - they still have curved sides.
In Hong Kong, the coins have different size and shaped ridges so that people with vision impairments can more easily distinguish between them! I remember the five dollar coin being big and round, the one was round and thinner too, while the two was more bumpy. And although the paper bill for $10 is more common now, the ten dollar coin was small than the other dollars while being quite thick.
Many other countries have something similar, but I do like the HK coins personally. As a kid I always played around with them and sorting them by thickness, aligning their ridges (particularly for the $2 coin), etc.
And indeed, none are sharp for the reasons you mentioned. Most coins have ridges though, as those are used to tell if some of the coin was sanded off for the metal.