Why any problem can be reduced to SAT is NP-Complete?
Indeed that is incorrect. All problems in $\mathbf{P}$ are reducible to SAT. In fact, even trivial problems (e.g., finite languages) are reducible to SAT. 

What is probably meant is that, if a problem in $\mathbf{NP}$ is such that SAT is reducible to it, then it is $\mathbf{NP}$-complete.