It's like the existing terrorists aren't smart enough to think of such attacks, aren't skilled enough to execute them, or aren't interested and are content with relatively minor (though still very tragic and media attention-grabbing) attacks.

It could also be that the agencies charged with keeping us safe are actually doing a good job at preventing those larger scale attacks, which makes sense for attacks like the 9/11 ones, which required a lot of coordination and cooperation between the participants and are therefore vulnerable to being spied upon and/or infiltrated, but not so much for other "lone-wolf" sort of attacks, many of which really don't require much skill and no cooperation or coordination with anyone else.

So I'm not sure how to explain not seeing any large-scale attacks by lone-wolves or even small terrorist cells except as lack of interest or knowledge on the part of the terrorists.