I understand your point entirely but for some people they'd rather pay someone for their time (and experience). Like I said, I don't want to spend 2 days researching what locks to fit, measuring them, ordering them, waiting for them to arrive, fitting them etc. Not when I can pay a fair price for someone to do it for me in an hour or so, and I can rest easy that it's been done properly and I haven't inadvertently bodged it etc.
I'm with you here - there's a few things I just won't DIY. I'd prefer to pay for the saving of my time and the piece of mind that the job will be done better than I could do.