Default directory for MySQL databases
That's a standard suggested location in the installation layouts to-start-with and usually people tend to follow the defaults.

Just to answer it short - keeping databases under /var/lib/mysql as such is not a "risk" as such!

There are considerations though. You can always change your directory to a different partition for better manageability and to take IO advantages. 
I've even seen people softlink /var/lib/mysql to a different partition to keep things simple-to-view at configuration level.