It is not impossible to build our own telescopes instead of buying off-the-shelf ones from a manufacturer. Amateur telescope making is a sub-group in the field of amateur astronomy where people build telescopes for personal enjoyment.
Some flat-Earthers claim that telescopes are programmed by the manufacturer to display fabricated images to the user. The possibility of making our own telescopes tells us that such a claim is false and ridiculous.
References
- Amateur telescope making – Wikipedia
- Telescope making – Wikipedia