Why doesn't G-actin concentration fall once microfilament polymerization begins?
The critical concentration is better defined as the "equilibrium concentration of the pool of unassembled subunits"1.

An analogy that might be helpful is thinking about a NaCl (table salt) solution — once you add enough salt some will start to precipitate, but the amount in solution will remain the same.

You can think of the solution as having the capacity to support a certain concentration of molecules (or ions) — below that concentration subunits will separate from the polymerized form faster than it can accumulate new subunits, above that concentration the subunits will stick together faster than they "fall off" the polymer.

For more a detailed molecular understanding of why this is so, you probably need to read up on chemical equilibria — for example Chemistry or even Khan Academy might be helpful. 

Reference:

1: Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., &amp; Darnell, J. (2000). Section 18.2 — The dynamics of actin assembly. Molecular Cell Biology.