Why? Because you won't earn all that much just by writing. Which means you'll need to put yourself in an economic situation where you can afford to write for a living. That means one of two things: find flexible work that leaves you plenty of time for writing, or get your expenses down--way, way down--so you don't need much money in the first place.
Wait. There's a third option. Do both. Find flexible work and get your expenses down.
For readers serious about achieving financial independence, I can recommend two resources. First, read the life-changing book Your Money Or Your Life.
Take these steps. In a matter of a few brief years, you'll find yourself in a radically improved financial position. I promise. Most importantly, you'll have the economic freedom to write.