There's a fairly basic contradiction in your posing of the question, which is that the more inert a metal is, the easier it is to purify by definition.
A metal that is abundant as oxides or salts but cannot easily be purified is a metal that is by definition highly reactive; it doesn't want to exist as its pure form because it is so much more stable as a reacted compound. You're not going to get a metal that's "less reactive" but "harder to purify", because these qualities describe opposing things.
The alkali metals, in Groups 1 and 2 of the Periodic Table, fit your definition of "easy to find, hard to purify" pretty well, by being the most reactive metals known to humanity. They react especially quickly to oxygen and to water, both of which are abundant on Earth's surface and are critical to human life, so an environment conducive to human habitation is going to be one that is very poorly suited to the purification of sodium, potassium, rubidium, cesium, lithium, calcium, strontium or barium. The purification of alkali metals typically requires not only high temperature, but water-free and oxygen-free environments (otherwise the metal will just oxidize or hydrolyze again as it cools). The Industrial Revolution was in full swing long before a scalable process for isolating sodium using electrolysis became commercially viable in the late 1800s. Similar work in high-temperature chemistry also gave us a viable method for aluminum refinement, turning what was a precious metal into an important infrastructure commodity in the early 20th Century, and the refinement of silicon, germanium and other "metalloids" that are critical to modern information technology followed in relatively short order in the mid-20th Century.
Alkali metals still aren't commonly seen in pure metallic form, even though sodium is one of the most abundant metals on Earth's surface, because the metals are too reactive for everyday use in their pure form. You can buy the stuff in kilo bricks from chemical suppliers, but about the only household use for it is to make sodium hydroxide for use as a (very strong) cleaning agent, and it's safer all around just to sell that stuff bottled at a useful but not critically hazardous concentration, than to have people dunking sodium metal into water in their mop closets.
The only non-handwaving answer to the question of making a relatively plentiful, inert metal difficult to purify is to require absolute purity. Gold is "24-carat" at 99.95% purity, but that's still one part per 2000 of something else. Last I checked, we've gotten another order of magnitude purer than that (99.995%), but absolutely pure gold has not been achieved on any significant scale. If you need absolutely pure samples of pretty much any substance on earth, you're paying a lot per unit mass, if you can get it at all.