Happy users, faster growing userbase, more e-store revenue, higher click-through rates, lower latency, better quality-of-experience, or more 7-day-returning users can be more important financial incentives than bandwidth. Humans are more than thousands of times more valuable than a computer.
When image quality and observed latency are at a level where users don't care even subconsciously (say, images look like camera originals and load in 100 ms), then bandwidth cost optimization may become a good 2nd objective.
This is a strange argument. The larger companies have plenty of people to work on all the things you mentioned plus bandwidth optimizations. Also, they’re rarely the same people.
The claim (from one of the people working on JPEG XL at Google) is that better image compression is going to reduce latency (or allow higher-quality images) and thereby improve other metrics besides bandwidth costs, giving internet companies even more incentive to adopt better codecs.
When image quality and observed latency are at a level where users don't care even subconsciously (say, images look like camera originals and load in 100 ms), then bandwidth cost optimization may become a good 2nd objective.