Two reasons: because I know my customers appreciate honesty and because it’s not actually possible to fully assess and understand all of what’s wrong with a piece of equipment without carefully inspecting it.
Therefore, any ‘quote’ you might be given by a potential repairer who hasn’t inspected and diagnosed the complete fault with your actual piece of gear is based purely on guesswork. My needing to inspect your equipment is designed for accuracy, not to lighten your wallet. I didn’t become successful by engaging in guesswork and I’m sure you don’t want me guessing when it comes to your equipment! This is a process that helps me to deliver reliable, reasonable estimates to my customers.
Think of it this way: do you think an expert mechanic could tell you over the phone why your car won’t start and what it will cost to fix without looking at it? Of course not, and once you process this, you’ll better appreciate the point I’m making.