As other answers have stated, it's the law. I'll just add some to the reasons for the law:

Every once in a while, a vehicle crossing the railroad tracks won't make it to the other side in time. The reasons include sudden drivetrain issues, getting stuck on uneven crossings, traffic ahead blocking the way, or simply not noticing the train. Signals can malfunction or be missed.

When it's a passenger car, including ones their owners call trucks, it's normally only a small incident. Cars are quick to evacuate, only having 1-2 passengers per door. They are also mostly sheet metal with relatively small rigid parts, so present little danger to a train, although that can change with high-speed rail.

Commercial vehicles are a much bigger deal. Buses specifically are slow to evacuate. Commercial trucks use long strong beams in their chassis, which can be a problem if they get pulled under the train. There have been dozens of fatal accidents involving train-truck collisions. While most train derailments are not Hollywood-grade catastrophes, the sudden stoppage can injure or kill passengers or overturn cargo cars.

The latest accident happened just a week ago: https://www.thelocal.de/20190508/at-least-20-injured-after-train-and-truck-collide-near-hamburg

As other answers have stated, it's the law. I'll just add some to the reasons for the law:

Every once in a while, a vehicle crossing the railroad tracks won't make it to the other side in time. The reasons include sudden drivetrain issues, getting stuck, traffic ahead blocking the way, or simple failure to notice the train, due to malfunctioning signals or inattention. This happens roughly 6,000 times a year in the US, or 16 times a day.

When it's a passenger car, including ones their owners call trucks, it's normally not such a big deal. Cars are quick to evacuate, only having 1-2 passengers per door, so only 1 in 10 train-car collisions results in fatalities. Being mostly sheet metal with relatively small rigid parts, road cars also present limited danger to the train. Most of these collisions go unnoticed by the public.

Commercial vehicles are more of a problem. Buses specifically are slow to evacuate, typically taking over a minute. Commercial trucks use long thick beams in their chassis, which can be a problem if they get pulled under the train. There have been dozens of fatal accidents involving train-truck collisions. While most train derailments are not Hollywood-grade disasters, with the cars just stopping as their wheels scrape over the sleepers, it's still likely to injure or kill passengers or overturn cargo cars.

The latest such accident happened just a week ago: https://www.thelocal.de/20190508/at-least-20-injured-after-train-and-truck-collide-near-hamburg