There is an interview with one of the project leaders on the James Webb Space Telescope which touches on this topic. He mentions that he hopes that the next space telescope is manufactured/assembled at least in part in space, the reason he gives are

• The difficulty and complexity of testing the equipment on earth so that we can ensure it works in space.
• The limitations on volume of the launch vehicle can impact the size of the final telescope, and in the case of JWST the complex heat shield unfolding and the systems to support that

Now as to why we don't already do this ?

• It will take us quite some time to build this capability, there is a lot of equipment and infra we would need to get into space to do something like this, and getting things into space is ridiculously costly.
• It will take us quite some time to learn how to do it efficiently and safely in a low gravity environment. Space just makes every step of assembly and manufacturing extremely complicated. Especially if the assembly is done outside in space walk. There is a Joe Rogan podcast with a former astronaut Garrett Reisman where they talk about how complicated space walks are. (link)