Q: Why does Safari gobble up so much memory?

Don't keep many tabs open, don't pin any pages, don't have too many pages open at once. Beyond that, there is little you can do except close the browser when it isn't needed. The more concurrent apps you try to run, the more third-party utilities and plug-ins you use the less memory you have available. RAM is a limited resource. The less you have the more you need to manage how it is used.

Of course you could buy a newer machine that can handle more RAM - 8-16GBs if you use current versions of OS X.

2 GB per page is not at all unusual. Many of even news sites I visit are more than that.

Why? Websites now a days are very graphics intensive, but there are also potentially dozens or even hundreds of ads, trackers, etc, that are loaded up with the sites. Even if these ads are not visible, they still consume storage.

One defense, is a good adblocker extension. There are many to choose from. I'm partial to the free version of Ghostery.