Not all sites enforce these limits, but major publishers are very aware of users' interests.
The problem is that if you visit an average website, there will probably be 20+ servers involved in serving the page, the javascripts, analytics, ads, etc.
One of those systems occasionally underperforms, which would not be a problem if everything but the main content was simply loaded asynchronously, possibly in the order of importance a website owner attaches to each content type.