In a nutshell, yes it is because of databases and the internet.
Think of the target audience for a volume like ECO/MCO - in the pre-database era, it was the only way to have a reference book that covered everything. So if you heard that your next opponent played the Catbum Gambit, you could look up a reasonable stab at the critical lines. Moreover, you know that lots of other players were using the exact same source!
Fast forward to today. The latest games and theory become available much faster. Any single printed reference volume will become outdated very quickly. You prepare for an opponent using databases (either online, or your local DB which is updated weekly over the net.) In order to be of value, books have to be more than just 'paper databases'. The one-volume Small ECO was published last in 2010 and the online review I saw from 2011 was already pointing out gaps/errors.
Therefore you still see a lot of repertoire books, where someone has gone to the effort to distil current theory and explain plans/pitfalls.
There are also some specialist subscription sites that maintain regularly updated info on the latest critical lines and evaluations in the mainline openings. So it's not so much "figure it out yourself" but more that the effort that used to go in the encyclopaedias now goes into online services.
Why aren't they updating single-volume encyclopaedias any more ?
Eco hasn't been updated since 2008,
Mco hasn't been updated since 2008,
Fco hasn't been updated since 2008,
Nco hasn't been updated since 1999, etc...
Is it because everyone now has engines and databases, so we can figure-out our own ?
Have opening encyclopaedias now become a thing of the past ?