It's probably been said on here but it's a tradition that began in a time when women were property. Engagement rings were an outward sign of impending ownership - this one is taken. Find another. You obviously know this though.
I believe the reason they eventually became to be high value is so that, in times when women couldn't own things or have income of their own, it was a type of insurance so that, of the man broke off the engagement, the woman could sell it and she'd have a bit of money behind her. Of course, women wouldn't break off engagements because everyone knows, all every woman wants is to be married! But he also wouldn't need the insurance in the same way.
Nowadays, some people like them and some people don't.
I got engaged recently. I had a budget in mind for a ring of about £300. In the end, we could find nothing in that price range that either of us liked (if at all tbh!) and the one we both liked was around £500. So we got that.
Most we saw that we liked were around £1500 - £3000, and he would have spent more if I'd wanted but I can think of things I'd rather spend £3k on that a bit of bling on my finger!! 😄
We did see a beautiful diamond solitaire for £68,000 mind... totally impractical but it looked lovely in the shop window!