brain is hardwired to respond to some simple shapes; circles, right angles, spirals, etc
this makes symbols that incorporate these basic forms more salient and thus memorable.
simple shapes are used for a number of purposes. People who desire power and understand psyche not only design lasting symbols, but given the age of humanity, they co-op previously used, powerfully associated symbols.
To an observer, this gives a novice the appearance of a natural mystic. The novice is associated with all previous uses of the symbol, though they can use plausible deniability to distance their use of symbol from anyone elses