That's because it's so great that everybody's already signed up!
Actually - the everyone’s already signed up part of what you wrote is somewhat correct.
Current enrollees are automatically renewed, so there is no reason to re-visit the website unless an Obamacare enrollee’s particular plan selection has been cancelled for 2019.
In any event, if it wasn’t set up to be on auto-pilot to auto-renew current enrollees’ I’ve got to believe that enrollment would be down 50% with the elimination of the individual mandate. It may just be that enrollees don’t know the IRS isn’t going to steal their refund in April 2019; or that the auto-renew keeps the enrollment out-of-sight/out-of-mind.
I’d be dropping that thing like a hot potato in favor of the old style plans that Trump has allowed to be issued again.