A lot of times, it is the case that emails used to create WordPress accounts are not meaningfully verified. Discourse depends a lot on the email address.
There used to be a disclaimer that before overriding the require_activation behavior one should ensure that they’re properly verifying the email one has used to sign up.
my most informed guess here would be that the option doesn’t exist as a checkbox is to prevent spam signups when WordPress is the SSO provider. However, a custom function can easily override this behavior.
add_filter( 'discourse_email_verification', 'wpdc_custom_disable_email_verification' );
function wpdc_custom_disable_email_verification() {
return false;
}