Looking at the the street view provided by the OP I would say that vertical profile constraints are at play here. The ramp needs a certain vertical clearance near its start and near its end. Rather then have the road return to ground level, the designers may have opted to use one vertical curve over the entire ramp that suited their design needs. The consistent vertical curve should ensure a comfortable ride for the user as well as provide plenty of sight distance to ensure they have enough time to brake if need be.
In addition to what has already been said about required clearance for over sized load, there may be a major industry on this route that routinely ships large/tall objects.
Similar to the need for formwork/falsework clearances, there may be an allowance for future maintenance or rehabilitation work to be performed.
Having done more than a few bridge inspection, I can dream and say they left plenty of clearance for an inspector to work under the bridge in a bucket truck over live traffic without the need for a lane closure! Pipe dream that one!