While it may sound easy to change a polymorphism (and even there only for a single base, don't get me started on larger structural variants), do not forget that you have to apply this one single change to 10 ^ 14 cells (or even just 1 in 10000 cells is 10 ^ 10), while evading the immune system and not inducing any detrimental other mutations. For a software developer it always sounds so simple just to fix a single spelling mistake but that really isn't it.

(+ obvoiusly the stuff from the other comments about how hard it is to determine the cause to begin with)