I was taught that dividing the numbers naturally into thirds:

I  II III IIII (all I) 
V  VI VII VIII (all start with V) 
IX X  XI  XII  (all contain X) 

Visually looks more "balanced" than having an extra V

When Roman numerals were in use by the Roman Empire, the name of the Romans' supreme deity, Jupiter, was spelled as IVPPITER in Latin. There was a feeling that using the start of Jupiter’s name on a clock dial, and it being upside down where it fell, would be disrespectful to the deity, so IIII was introduced instead.

https://newgateworld.com/blogs/style/should-it-be-iiii-or-iv-on-a-clock-dial#:~:text=When%20Roman%20numerals%20were%20in,so%20IIII%20was%20introduced%20instead

I would have thought it had to do with aesthetics. I would have never guess it had to do with roman religion.

VI would be IIIIII which is severely over-wide. The balance is really against VIII and XII, you don't want one leg of that triangle to have a limp and IIII makes IV just a bid wider and chunkier to provide that balance. "Symmetry" was probably a poor choice of word this isn't a mathematical thing but perceptual, those three points being equal visual weight evoke an equilateral triangle standing on its side which says "yep this won't tip over, ever", because, well, things shaped such don't and the back of our head instinctively knows. Thus you get a sense of stability, and I guess this is a good example of why artists often sound like mystics or plain nuts ("this song tastes of strawberries").

The IVPPITER explanation definitely also makes sense but it doesn't explain why people continued to do it after standardisation on IV in arithmetic and the fall of Roman paganism.