This is for trains in the UK that are near London, I don’t know if it’s the same in other places.
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Because of weather. People don’t like to be cold or wet. Not to mention protection from other things such as mechanical failures. If something comes loose, you at least have a small amount of protection that could help.
The train cars can be disconnected and reconnected in various ways. Without doors, the train configuration would be fixed.
Having a articulated connection is much more complicated and expensive to build and maintain.
There are several reasons. Train cars are often disconnected and reconnected to each other. So you need a door where there is no connection or it is connected to a non-passenger car. The ability to disconnect also makes it very hard to seal the cars to each other. So the space between the doors, between the cars, tends to be partially open to the elements through gaps in the seal. Without the doors the inside of the car would get quite hot/cold, dusty and noisy. But even within a car there might be doors. These are primarily to make sure that noise from the passengers do not spread to the entire car or even neighboring cars. Just imagine having to listen to a crying baby from the neighboring car or juveniles taking the train to a party.
If by open you mean no doors and a continuous corridor between modern carriages:
Noise attenuation and its cost. The door is soundproof. Outside the train there’s 90 decibel.
The connection between carriages is where the noise gets into the wagon.
When necessary, like a metro train, you can build a fancy flexible joint but it cost a lot. Having a continuous space allows people to board and walk to another wagon easily, so they can look for a free seat. When you are expected to stay seated for a long travel, on an already reserved seat, there’s absolutely no benefit into spending that much money to have a connection no one walks through. Money better spent in a good door to keep even more noise out.
There are trade offs, and some vehicles have something in between an open path and a door path. Like, having light automatic doors with a semi soundproof connection.
Old school trains did prefer doors so they have a quicker time to assemble the train. Let’s say it was rush hour, they could add one or two extra carriages in minutes. Flexible joints can’t be disconnected or connected outside of a maintenance shop.
New school of thought is to make fix lenght trains, and change how many trains per hour you send down the route to compensate for rush hours. In this case, you can permanently connect carriages with flexible soundproof joints, as you don’t ever need to change train lenght.
Some DMU/EMUs in Austria have seemless passtrough between the carriges but the old Eurofima wagons still have those doors.
The difference between modern DMU/EMUs and Eurofima wagons is the flexibillity of arranging the wagons you need to make a train.
The doors allow for much greater variety to configure fancy things like portion working, where one part of the train is decoupled fom the rest for departure to a different destination. The doors allow for those things to be more flexible.
You can also do portion working with DMU/EMUs, but you are limited to those units.
Let’s assume that you depart from Vienna and head to Wels for dividing up the train into two parts each heading to Salzburg and Passau. With DMU/EMUs, you can only divide in the middle, witch doesn’t make sense when you know that more people want to Salzburg than to Passau.
The solution?
Using seperate wagons with said doors that you can arrange to yours hearts content.
The doors help in that regard, that they can be locked up, so no one can fall back out/no one can sneak into the part heading to Salzburg when they bought tickets to Passau.
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