I believe log4net (and others like NLog) are better than the built-in, but the built-in has one nice feature — In visual studio, the output window conveniently shows the log messages if you use Trace.WriteLine or Debug.WriteLine.
Note output window isn’t the console.
Can log4net write to the output window? See http://angledontech.blogspot.sg/2011/08/direct-log4net-output-to-visual-studio.html. A bit of work. When I start a new project and have no time to set up log4net, then I use Trace/Debug to dump to the output window automatically, by default.
Reason2 – some projects may use the builtin not log4net.
Reason3 – some projects use other logging frameworks, so you may need to learn multiple logging frameworks. If you are lazy and you can decide, then the builtin is a basic solution always available.