It's easily possible to grind your own big telescope mirror. A big lens is like trying to build a big refracting telescope -- extremely expensive. The largest refracting telescope ever made is 100cm (40 inches). Mirrors are up to 10 meters.
Most of the telescopes are extremely dark compared to "normal" camera lenses.
The brightest that I remember was the so called "camera Schmidt" with f/2.8.
Normal Newtonians usually are in the f/4-f/5 range.
The cassegrain and derived are around f/10.
The big aperture in the telescopes is needed for the angular resolution, certainly not for the brightness.
Yes, the resolution is given by the diffraction limit of the optics that is dependent by the diameter.
But resolution is different from brightness.
If you tried to take a video of the same scene with a telescope chances are that the final result would have been darker, even using the same camera.
Granted that it would have had a much much better resolution (but a much much smaller FOV)
So you think astronomers are too stupid to trade resolution for brightness? Hint: downsample your image: you won't lose too much as long as the dark current isn't too large.
They have completely different use cases.
In one case you are making a video of a scene with a FOV of several tens of degrees.
In the other you are observing objects with a maximum size of a couple of primes and with features measuring infinitely small fractions of arc seconds.
It's easily possible to grind your own big telescope mirror. A big lens is like trying to build a big refracting telescope -- extremely expensive. The largest refracting telescope ever made is 100cm (40 inches). Mirrors are up to 10 meters.