> Ubuntu 16.04 doesn’t have a built-in dictionary or thesaurus
What OS has build-in dictionary? There is no need to preinstall all the programs. Even if it had one it would be just one more thing to remove and install another dict (everyone has different needs).
NeXTSTEP came with the Digital Librarian, which included a Merriam-Webster's Collegiate Dictionary and a thesaurus. Beginning with Mac OS X Tiger, macOS has shipped with dictionaries. I make heavy use of the Japanese dictionaries.
Unfortunately I've had a difficult time finding offline dictionaries for Windows and Linux. Microsoft used to offer offline dictionaries (Microsoft Bookshelf had a dictionary, and I believe there was an Encarta dictionary as well), but last I checked, Microsoft no longer offers an offline dictionary. It used to be possible to buy a CD-ROM version of the Oxford English Dictionary, but last I checked there is no longer an offline digital version of the OED. As far as Linux goes, the dictionary software that I've seen largely relies on either public domain dictionaries (such as the 1913 Webster's dictionary) or open source dictionaries (such as Wiktionary), but I'm unaware of any Linux dictionaries that use commercial dictionaries.
I'm willing to pay for good offline dictionary software for Windows and Linux (bonus points if I could find good Japanese dictionary software); that's one of the things that keeps me on the Mac, actually.
What OS has build-in dictionary? There is no need to preinstall all the programs. Even if it had one it would be just one more thing to remove and install another dict (everyone has different needs).