The simple truth that most software engineers don't want to hear is - people are quite conservative in this. If it works once, most folks would be extremely happy to keep using it for next 20 years in exactly same way, as it is. Bug-less has higher priority than shiny (which is highly subjective) and constantly changing.
Signups/purchase flows are meant to trap users and keep them from escaping--not create a smooth, easy to use experience
Unfortunately this tech oligopoly landscape doesn't leave users many ways to express this preference, but I often wonder if the culture will change at some point in the future as the industry matures.
An aging western population might have an influence here as well, in the long term. Older folks have less tolerance for having to relearn how to do the same thing they already knew how to do for the umpteenth time.
AirBnb lost billions of dollars last year, and it has rarely made positive quarterly earnings. Based on your example, the engineers are highly overpaid as there efforts rarely generate a profit. They are being paid by funds from speculators (stockholders and venture capital) that are betting on huge profits in the future.
In reality, these engineers are being paid due to labor market forces and extremely focused recruiting policies (eg only hiring those with degrees from top engineering schools or poaching workers from other companies that have similar policies).