A huge chunk of the database was posted around that time (the site was launched in 2011) so unless someone has gone out of their way to update it then that's all the biography will show.
Biographies are meant to be concise, long ones may be too bulky.
Updating their biography with crucial information is important but too much information is generally unwanted from what I've seen. After all, the dramas, movies, and shows they've starred in tend to tell most of the story already.
Here's what the mods posted about the guidelines: https://mydramalist.com/discussions/support/17285-guidelines-editing-and-submitting-new-dramas-movies-and-actors
Also, do note that edits are not posted automatically. They have to be first approved by one of the Approval Staff before going live, which from my experience can take a couple of minutes to about 2 days.
