They do in a lot of metros, although it's often very hard to retro-fit. The DLR got it at build-time. [1]
One of the big issues is 'degraded working' where there's been some sort of failure (or perhaps adverse weather conditions) which means trains have to operate on vocal authority to proceed on sight. The problem then becomes as hard as driverless cars - you have to know when you've reached milepost 70 or the next set of points by sight and it's really bad if you miss them. Arrival to and dispatch from stations without platform-edge doors is also tricky and human staff are often required to manage the process.