Lets just bolt the pole to the bridge and hope it holds

Oh god.

what excactly is missing here?

Nothing's missing. When you have the pole attached to the bridge the bridge is the foundation. Same with street light poles. You don't need a 4ft deep foundation or whatever because the pole isn't buried in the ground. The whole bridge is the foundataion.

Looks like the bolts go into the I beam?

They might. Even if they don't it shouldn't be a problem unless they use poor quality concrete.

You are right. The pole looks older.