Why is it so predictable? I know investors don't care about anything other than returns, but what returns do you get when you tank companies in such diametrically opposed ways from how they've operated before?

There are different kinds of investors involved in an IPO process.

The investors that have pre-IPO stock and who are looking to sell as quickly as possible, gain from IPO pops, so it's in their best interest to maximize short-term profits. That group of investors include a lot of the existing investors, and in almost all cases the founders and C-suite of the company.