With regular phone service, a wire is effectively plugged together all the way from you to your destination and is dedicated to your use (yes, it may be multiplexed with other connections on the same fiber part of the way). That usage is probably very light compared to the potential capacity of the connection, and there must be enough capacity to handle peak loads, so it's underutilized most of the time.

Internet transmission is "connectionless". The transmission is chopped up into "packets", the packets are addressed and "tossed into the cloud". Routers pass them to their destination by the best open path, and the routers can cram in the maximum traffic the wires and fibers can bear.