The short answer is the directors like them! More rationally, bikes are easy to get hold of and small enough to work on but not so small that they’re fiddly. They are human scale machines. There is also a ready demand for good used machines.
 
The tools used are mostly small, blunt hand tools, so there is little danger of serious injury (grazed knuckles are inevitable however).
 
One of the best things about bikes is that in principle they all work in the same way and can be fixed with the same tools, yet there is also a great deal of small variation which keeps the interest up in the more experienced members (and staff). This is particularly the case with old bikes, which can throw up interesting challenges. The jobs that need doing on a typical day in the workshop vary from the very simple cleaning, which anyone can do, to wheelbuilding which takes a lot of practice to be good at.