The large telescope mirrors aren’t single parts even in terrestrial telescopes. We have to assemble and calibrate them on their isolated mountaintops, starting with the giant support structures intended for suspending the mirrors against the pull of gravity with minimum distortion regardless of orientation of the telescope.
We then add active optics to compensate for atmospheric distortion, alter airline routes and city lighting to help improve conditions, and then pray for no clouds on the nights we want to observe.
Constructing telescopes in space will become a reality within the next decade, while terrestrial telescopes currently in planning will be finding it harder and harder to navigate the mixture of light pollution, accessibility and indigenous rights. I expect there are terrestrial telescopes already in planning and even at early stages of construction which will never be finished. Between the ever-growing forest of red tape and the ever shrinking cost of building space telescopes in orbit, there will be an inflexion point where it makes sense to dedicate the remaining budget of a terrestrial telescope project to building its spaceborne replacement.