(This is probably more a "copyright" question than anything, but I have not found an appropriate StackExchange site for this kind of question.)
Why are fonts like Akzidenz-Grotesk, that were made way back in 19th century (Akzidenz-Grotesk was made in 1896), still copyrighted and not in the public domain?
It's got to do with the legal distinction between a font and a typeface. Fonts being the digital implementation of an original typeface design.
The Akzidenz-Grotesk typeface may be over a hundred years old and out of copyright, but the font that happens to share its name and is obviously based on it, is a new and distinct legal object, subject to its own copyright period.
You're (probably) within your legal rights to create a brand new font that also happens to be slavishly based on the original typeface, as long as you create it from scratch and don't use the name 'Akzidenz-Grotesk' (hence, fonts like MS's Book Antiqua being a blatant ripoff of Palatino and other similar cases).
There are many versions of Times, Garamond, Caslon and other typefaces sold by different foundries. Each implementation is copyright to the publishing foundry, even though the name is the same. The design is in the public domain; the implementation is not.
Further to the answer by Heroes182, noting the distinction between a typeface and a font, it should be noted that typefaces are unprotected by copyright in the US (but are protected in the UK) which is why their glyphs can be copied into a new font. Either way, the shapes are up for grabs.
An electronic font file, on the other hand, is interpreted as a computer program by US copyright law, so is protected against copying. This means that you have to use a non-digital method to reproduce the font if you want to make your own version.
Finally, a company selling a particular font under a particular name (like Palatino) gives rise to a trademark on the name. It's that protection that stops other fonts being called the same name (cf. Microsoft's Book Antiqua).
See the NRSI article Font Licensing and Protection Details for more information.
Some additional remarks:
Works that have been created in the 19th century can still be realistically copyrightable, because the relevant date in many copyright laws, in particular the German one, is the death of the creator. As of now, a work enters the public domain after 70 full calendar years have passed since the creator’s death. So if the creator died later in 1945 or later, their work may not yet have entered the public domain. And living 46 years after having created a font is not that unlikely.
The analogue Akzidenz-Grotesk has a convoluted and not fully known¹ history and been modified many times between ca. 1880 and 1985. Thus, while the early versions and styles probably are in the public domain, the younger ones aren’t. Most of the work since 1950 has been done by Günter Gerhard Lange, who died in 2008, rendering his work copyrightable up to 2078 (if the copyright laws are not changed until then).
¹ See, e.g., this source in German.
In the US, design patents are issued for a maximum of 14 years. Thereafter, the designs are public domain. Other countries have similar laws and negotiate international agreements.
In the US and elsewhere, trademarks may be renewed indefinitely, and revival developers have been sued for infringement of typeface names.
The Library of Congress has never issued copyrights for type designs per se. Even so, digital fonts are automatically protected because when printed, the code pattern forms an original creative work.
