At the risk of sounding too Clarke and Dawe, it is very rare that a ship loses power and control, and somewhere it could hit something important, and hits that thing, and the thing is apparently so fragile that it just falls to pieces. It's been there for 46 years, and the Port of Baltimore currently sees an average of 53 ships in and out per month, so about 3.5 big ships under the bridge per day. That's a lot of passages over the years without incident.

Cause then we would have to hire more people to tug all those ships in and it would be less efficient.

Not very profit margin of you to suggest that.

Politics.

"More tug jobs? Not on my watch!"

Why aren't ships that size required to 100% be escorted by tugs???

They likely were, but there are limits on how fast even a group of tugs can influence a ship many times their size/weight/mass.

The laws of physics still apply.