Because there have been several testnet merges (including a shadow merge of the mainnet) that have been successful and identified issues that have been fixed.

I'm sure they've found and fixed issues. are you sure they've found and fixed all the issues? if I were a hacker, and I found a juicy bug in PoS, I'd sit on it until merge. either make money by shorting ETH and then wreaking havoc, or sell it to someone else so I don't get my hands dirty.

exactly! hence why I think this must be the most harrowing software upgrade of all time.