Because anyone who complains is given this treatment. We don't complain to everybody we just bounce.
On the other hand, Gmail regularly takes 10s+ to load and yet most users are glued to it.
If you're a SaaS business, your marketing page is probably not your app. The people using your app care about usability, and page load speed is rarely the driving factor. Especially with SPAs.
Better yet, ask your actual customers.
Except on forum's. We complain there :P