The problem with 3D Secure is that the merchant can unilaterally decide not to use it, which defeats the whole purpose of 3D Secure.
I tend to associate ignoring 3D Secure with Stripe. In the name of "less friction" of course.
non-3DS payments are trivial to chargeback, at least in the EU
> the merchant can unilaterally decide not to use it
If they do so, they are telling the card issuer that they are happy to be on the hook for chargebacks/fraud. It's not an decision without consequences