Make no mistake. Snap's (relatively early) IPO wasn't to raise capital for further expansion, it was to ensure founders had easy access to liquidity in a service they knew wouldn't thrive over the next 5-10 years.
Another reason for turning down a buyout situation is the terms. They (Evan and the VCs) would not have such favorable terms as the IPO where they received substantial stock grants, liquidation preferences and retain absolute control over the company without requiring a large (any) financial interest.