Because of many factors. You can't see how well they hung the boiler for a start.
I've been to big radiators that have fallen off the wall, fixed with 1" 10's. You don't know if they flushed the system properly, you don't know if there are any hidden comp joints on the gas line, unless you can inspect it all.
I once found a drop on a gas line, when I tracked it down, it was an old green joint, that the installer had forgotten to blow in. No solder at all, just the flux stopping a leak.
I like to know its my work that I'm putting my name to.