Most modern bridges do take into account the possibility that there will be collisions. But it is also very hard to protect against a collision with an object that weighs 100k tons or more. By the time the boat is crashing into the bridge it’s too late. The solution is to design better systems around preventing things from getting to that point.
A good analogy is airport security. We are very bad at keeping weapons off planes. The TSA routinely fails to find knives and guns. The reason we haven’t had any more 9/11s isn’t because we made buildings airplane proof or because TSA doesn’t let box cutters through (they absolutely do). It’s because we learned to pre-empt coordinated well organized attacks using intelligence, and we hardened physical security at the cockpit door (I’m guessing mostly the latter).
You're forgetting the REAL reason airplanes are basically not hijacked anymore: Flight 93.
It used to be, if you threatened to blow up an airplane, the passengers would largely choose not to confront them, because the SOP of hijacking was to land somewhere and negotiate. Indeed this desire to not rock the boat actually turned one hijacking by drunk teens into a catastrophe when the hijackers were too stupid to understand that a plane couldn't just fly to Australia without picking up more fuel, Ethiopian Airlines Flight 961.
9/11 changed that risk calculus for passengers. Now everybody knows what hijackers COULD do, and are willing to endure some danger to prevent that, to the point we've kind of overcorrected and people are yelling at and harassing fellow passengers for non-issues.
Not just that, but also to rethink how we reinforce them in major waterways or even minor but navigable waterways, if any heavy enough boat can just crash into a support column and take the whole thing down... albeit accidentally in that case.