Partly due to a few unknowns on whether everything will fit, partly liability if something doesn’t work and partly them just not liking that sort of thing
This. Brakes can be a pig to set-up well. You don’t describe the state of your bike; if there’s a BB already in there it may be reluctant to move. You may be supplying all the parts, but may be missing the pieces that fix said parts to said frame, or pull said part to make it work. Frame and parts may not even be happily compatible.
You’re paying for all the ‘what-ifs’ that will almost certainly arise.
I design signage for part of my living and work on the premise that everyone is stupid. Similarly I have worked as a mechanic and a guide, and used a similar premise. I have been that stupid person buying car kit and stuff for workmen turning up to do house alterations. However, if you assume that everyone is stupid you can only be pleasantly surprised. And what’s more, if you’re not stupid you may even be pleasantly surprised when it doesn’t cost as much as you thought it would*.
*Gifts of biscuits / cake / beer can also help in this particular instance.