It was a common old English naming practice to combine two words to form a name. It was pretty similar to other Germanic languages in that way. So you get names like Ælf-ræd (Alfred) which means elf-council. Incidentally that was a great name for a bookish supposedly somewhat sickly kid.

So the regular use of Æthel- as a prefix is because it was seen as a valuable name to give a kid. Æthel- means noble, in both the “good” and “aristocratic” senses of the word. So you get a whole host of names using the prefix. Æthelwine (noble friend), Æthelred (noble council), Æthelstan (noble stone aka gemstone), Æthelwulf (noble wolf), Æthelthruth (noble strength), and so on and so forth. It’s etymologically related to the continental Ael-, Al-, and Edel-. So it’s interesting to consider had George VI of Great Britain reigned a thousand years earlier, he would have likely kept his birth name Albert, and may have been known as the old English equivalent Æthelberht (noble famous).

It’s important to note that the Æthel- prefix was an aristocratic prefix. So it’s use was limited to the aristocracy. It also tended to go to older children, either men who would be expected to have titles or women who were to be married off to noble men. Thus it’s a touch ironic that Alfred the Great was the only one of his siblings not to get the Æthel- prefix. His older siblings were Æthelstan, Æthelswith, Æthelbald, Æthelberht, and Æthelred. Clearly their father Æthelwulf expected noble accomplishments from all but Alfred.

So short answer. Æthel- was a very common prefix in Old English, but only among the nobility. As TV shows tend to focus on the doings of the upper classes, it’s not unreasonable that they should have a boatload of Æthel- names to confuse modern audiences.