As you can see from the first deleted scene from Star Wars: Revenge of the Sith, the simple answer is that humans don't generally understand Artoo although they may have a basic understanding of "spoken Droid". Note that while Anakin is able to reproduce the sounds perfectly, his comprehension is limited and he is relying on the communicator to provide a textual translation.
Anakin: Artoo, where are they holding the Chancellor?
Anakin: No text
[Anakin hits the communicator]
Anakin: It's broken. Can you understand what he's saying?
Obi-Wan: I'm not a protocol droid
Anakin: I'm pretty sure that beep [he perfectly reproduces Artoo's "speech"] is down.
You will also wish to note that this is exactly the same way that Luke communicates with Artoo in the cockpit of their X-Wing, via a speech-to-text app.
"Most people" don't speak droidspeak. Only those that work around droids bother to learn it, and even then inexpertly.
Notable characters who can understand droid (without a translator device) are;
Luke (in his later years)
“Artoo?” he asked, brightening, and a moment later the blue-and-white
astromech rolled into view, chirping and whistling at length.
“Yes,” Luke said. Decades of missions with R2-D2 had left him
reasonably fluent in droidspeak, but the astromech’s list of
accusations was both lengthy and highly specific. “No, I—yeah, it’s
R2-D2 squawked derisively.
“Hey, sacred island,” Luke said. “Watch the language.”
The Last Jedi: Official Novelisation
To most people, the beeping would have sounded like mechanical
gibberish. But Rey was good with technology and had studied the
communication patterns of droids. And that droid was especially
eloquent—particularly in his choice of insults toward the Teedo.
The Force Awakens - Rey's Story
BB-8, secure in the astromech socket behind the cockpit of Poe’s X-wing, burbled a question. Plugged into the fighter, the droid’s binary-speak was automatically decoded and displayed on the console, but Poe didn’t actually need to read the translation to understand what the droid was asking.
Star Wars: Before the Awakening