I wouldn't say they're "typically" all beef. When you go shopping for something like hotdogs for instance, if you just pick up a package of "hot dogs" or "weiners" they'll typically be a mixture of chicken, turkey, pork, and maybe beef. For instance:
https://www.oscarmayer.com/products/00044700000045-bun-lengt...
Some people really look down on these kinds of products though and see this as a "mystery meat" of extremely poor quality cuts. The whole phrase of "knowing how the sausage is made" comes to mind. So that's then in comparison to "franks", which are often 100% beef and are often seen as higher quality:
https://www.oscarmayer.com/products/00044700073377-jumbo-bee...
Are they higher quality? Eh. They're probably about the same in the end. One big thing pushing up the popularity of beef franks over pork/chicken/turkey mix is an impression of Kosher hotdogs being of very high quality during the time when meat packing was really bad, so lots of even non-Jewish people will often see Kosher dogs as superior. Which, for obvious reasons, have beef and not pork.
Brats are usually pork. For example:
https://johnsonville.com/products/original-bratwurst/
This is then true for other kinds of common sausages as well. Kielbasa is usually a pork and beef combination, maybe with some chicken or turkey:
https://www.hillshirefarm.com/products/smoked-sausages/00044...
They even make a point to say "smoked sausage" versus "beef smoked sausage".
https://www.hillshirefarm.com/products/smoked-sausages/00044...
https://www.hillshirefarm.com/products/smoked-sausages/00044...
Things like chicken sausages are also quite popular.
https://www.aidells.com/products/lunch-dinner-sausage/007640...
And one can't forget things like andouille sausage which is quite popular in a lot of Cajun dishes.
https://www.aidells.com/products/lunch-dinner-sausage/007640...