Excavators are also designed to work on dirt/mud or over locations that can't have high surface pressure point loads. Like a car, all of the weight is on 4 points (the tires). With the excavator, all the weight is distributed along the entire track for the most part. So even though the excavator likely weighs 3-4x more than a car, it's probably less stressful to the bridge overall.

When you drive over a bridge that is rated for "5 ton" or something like that, the concern isn't the total vehicle weight, but the axle weight. Bridge design is fucking wild. They even more or less set the requirements for the trucking industry trailer designs. They set the requirements for maximum loading and axle spacing.

Here's some cool info for the folks that want to know more

https://ops.fhwa.dot.gov/freight/publications/brdg_frm_wghts/