Metals start to get weaker at relatively low temperatures. https://www.engineeringtoolbox.com/metal-temperature-strengt... We want to extract 100’s of MW of heat which presents several issues.

You need a thermal gradient to move heat from the surrounding rock to the water your pumping in. The higher the initial temperature the larger the difference and therefore faster you can extract heat. Aka even if the working fluid is at a constant temperature you can simply pump it faster.

Also, Carnot efficiency limits means you want the hottest steam you can handle. Further, you have various inefficiencies such as losing heat as water moves back up a borehole.