You can have button to switch it off. It's even in post above yours. If your question is why Toyota/Subaru didn't implemented as button in stock car .. answer lies in country below yours
. Big vendors don't need yet another class action suit for "making it too easy" from crashed & died hoons to fullfil wishes of small part of anti-nannies purists. As for myself .. it was hard to get PD right during first PD usage times on track to engage it .. but rather soon i learned sequence by heart & can quickly perform it if/when needed in track day, hence i won't shell any $$$ for buying that "button". There are many enough cars where there is no PD, nor even with TC off (which results in some funny stories from friends/colleges about eg. starting uphill on ice/snowed road). I'm happy with what we have. I'd wish for VSC to be tunable, but that's just wish, not must-have, as potential of it is Very Big (for example, with tunable one could learn control by gradually upping limits, letting it correct only very biggest mistakes that would result in spinout/crash anyway, one could actually drive faster, due electronics in potential being able to react much sooner then human (not that good for learning though), one could change the way it engages (eg. on new miata i found it way less unpleasant even if left on)).