To be fair, groupthink in web design has generally led to worse performance with 100kb pageloads to show 1kb of content, and this blame generally does not reside with the product manager.
I'd say that the incentives are aligned in a way that produced the outcome of website bloat. The churn of hardware enables it, service providers have different priorities, and frankly, 99.99% of the users also have different priorities. So, no one is really striving for less bloated pages - including, most importantly, the product managers, whom have all the say of how developer capacity is allocated.
