I guess it is all down to routine and perception. Birds don't tend to be aware of the speed things travel at. I guess train drivers see this on a regular basis since they can't swerve to avoid animals / people.
When you drive the sort of mileage I do each year you get to see the strangest things. Since I started on our project 9 years ago I have covered over 180,000 miles commuting and business according to my mileage spreadsheet.
During that time I have seen numerous birds splatted by trucks and coaches. Newer coaches have huge mirrors that prove particularly effective at shredding birds and converting them into a cloud of feathers and a solid lump that goes flying across the road.
2 memorable incidents :
One morning just east of M4 Junction 13 a deer jumped the fence onto the eastbound carriageway, sprinted across the 3 live estbound lanes and then jumped the central reserve. The car coming the other way in the "fast" lane hit it and wiped out. At the speed we were travelling we were already hundreds of yards down the road but we called it in anyway. The driver was seriously hurt but survived the immediate accident and was taken to hospital alive. Don't know if they recovered or not - I hope so but it was a huge accident.
Some weeks later westbound at M4 junction 12 the cars in front started to slow down and swerve about. We passed a truck parked on the hard shoulder facing the wrong way and a ton of debris all over the road. Afterwards from the radio reports and going over what we had seen we realised the truck had punched through the central barrier from the other side and crossed 3 lanes of live traffic at 16:30 on a Friday without hitting anything (somehow). No wonder the driver looked a little pale when we passed him

From our perspective we got away lightly since they shut the road for 4 hours shortly afterwards.
I just goes to show - when your numbers come up that is it.
One of my colleagues was driving up the M5 one day from a diving expedition when the car in front hit the central barrier and shot back across the road before coming to rest on the hard shoulder. He stopped and went to investigate. The windscreen was gone and blood everywhere. The lady driving was totally distraught. She turned out to be unharmed (physically). What had happened was a pheasant had been on the road and got hit by another car and bounced up into her windscreen - shattering that. The ex-pheasant was spread all round the inside of the car - including all over her - hence the distress. Her car was recovered and the RAC arranged for her to be taken home after a visit to the local hospital for a sedative.
Cheers
John