You're right that track circuits are simple and relatively reliable, but they are also inefficient. Having to retain sufficient safety margins while being unaware of the real location of trains means that there is excess "dead space" in each signalling block that could be used to drive more trains over the existing infrastructure. It's what happens with systems like the DLR in London, which uses an advanced moving block system.
There is a tradeoff of course, but it's not at all clear that the simplicity of track circuits outweighs increased utilisation of the track.
There is a tradeoff of course, but it's not at all clear that the simplicity of track circuits outweighs increased utilisation of the track.