Exactly and its not entirely their fault. Web developers are notorious for implementing fancy design concepts without considering the effect on page load times.
Respectfully I'd say it IS their fault as much as anyone else's. The site's directors/owners can specify that a page load in X amount of time and that is that.
Yeah they could and maybe they do nowadays, I used to work for one of those web design companies years ago.( I wasn't one of the designers, I mostly just took care of the computers and the network) Our clients usually only had a vague idea of what they wanted their site to look like so it was mostly up to our design people to tell them what they needed to have and they were all about being creative and not much about technicalities like bandwidth and load times lol. It wasn't really part of my job but it was usually down to me to live test the sites they were working on and tell them about those kinds of problems.