They exist, many new lines or expansions are highly automated. Retrofits are hard for two sets of reasons:

Political: union strikes (meaning if you want to automate a line you may have to accept it being closed for the duration) and public outcry over a reduction in jobs.

Cost: You often only get the benefit of the simpler domain if you can implement the whole system from scratch, if you have to deal with the existing signalling, engines, (and human-driven trains) that's a complex problem, with massive capital outlay.