You said "look at any dictionary", so I did. I notice you can't provide a link to a single dictionary that supports you, or even name what dialect supposedly doesn't have "etc."
Etc. is an abbreviation for etcetera. Correctly signifying contractions, abbreviations, and acronyms is far more commonplace than using the correct dash. Almost everyone would have learned about shortening words in high school; many people leave university without ever having heard of an em dash.
Etc is also an abbreviation of et cetera. Only Americans put pointless dots everywhere.
This is all stuff you learn in school. Punctuation isn't obscure or niche. You may not have learnt about semicolons or em dashes in school but you should have and I did. As did anyone that has ever read a novel. There are two semicolons on the first page of the first Harry Potter book, a novel read by approximately every child of my generation. There are loads of examples of the proper use of dashes and other "obscure" punctuation marks in any professionally typeset text.
actually it's "etc."
(I wouldn't usually be a pedant, but if you think the difference between "--" and "—" matters, you should probably try to get the basics right too.)