Stars, intense gravity, collisions, etc. produce natural light in the radio spectrum that is captured by radio telescopes.
Small asteroids and comets produce no radio waves unless they are being heated by an atmosphere or a star.
Are we going to build giant radar transmitters and flood Earth's local neighborhood with radar waves 24-7 to detect silent almost invisible space rocks and ice boulders that might strike Earth?
That sounds like a hugely expensive and complex project.
A Radio Telescope is nothing more than a giant Radio Receiving Antenna, and the Green Bank Telescope is 100 Meters across.
An antenna can not only receive, it can also TRANSMIT.
Connect a Radar Frequency TRANSMITTER to it and it becomes a Steerable Radar System, without having to build a new dish.
And Radio Telescope can be modified to do this.
If they really wanted to, Aracaibo could be rebuilt and refurbished to do this as well, although it isn’t as steerable................