Sails are very complex creatures. There are many variables when buying one. Certainly the cloth is one item but even that can be hard to compare, with the limited information most lofts give you. (for that matter some don't have a clue about some of the more technical issues of the cloth) There are many different types of cloths than come under the same broad name. (have you ever tried to compare bed mattresses?)
There is also the issue of where you normally sail, local conditions, how you sail (racer, cruiser) how much wind you are comfortable with. How often you sail...
There are many dimensions to measure as well. Your old sail measurements. Also,whether you were happy with it. Pictures even of your old genoa while under sail can even be of help in designing a new sail. A minimum of about half dozen (closer to a dozen) measurements of your boat and it's rigging.
The list is almost endless that a good sailmaker will ask you.
On the other hand 75% of people won't know the difference and can buy something off the shelf with the "standard" measurements. And admittedly, some people go a bit overboard.