Because that’s hugely inefficient compared to a subway network.

I live in Paris, France. 100% of its inhabitants have at least one of the 300+ subway stations under 1 km (0.6 miles) of their home. The subway system transports 5M people every single day. That’s twice the city population. That’s also 1.9B per year. During the peak hours you can have up to one train every 90 seconds for a capacity of 700 people each.