Why does python use Karatsuba algorithm for its multiplication? Isn't FFT faster?

I assume by "FFT" you're referring to something like Schönhage–Strassen. The answer is likely that this algorithm, while asymptotically faster than Karatsuba, is more complicated and only achieves this advantage on very large numbers due to larger constant factors (Wikipedia quotes multiple sources that cut over to this algorithm only when the multiplicands have tens of thousands of digits).