They're not idiots, they just don't give a crap about waste until it affects their bottom line. (Most humans seem to operate on this principle)

A lot of the business-related bullshit is a response to advertising and sales. But a lot is also lazy tech people slapping pieces together until it does a thing, and they don't care that it loads 100x more data than it needs to, because again, doesn't affect their bottom line.

Traffic costs must be pretty low, or nobody's watching that line on the AWS budget, or the CDN is eating the cost.