Why does ptosis repair cause eyes to become drier?

I think your concerns are very valid.  Now I also understand your surgeon's perspective.  The bottom line, successful ptosis surgery will open the eye more and it is more likely that your dry eye symptoms will be greater once this occurs.  My best advise is to listen to your own intuition.  Have ptosis surgery when the situation reaches a point where you can accept the possibility of having a more symptomatic dry eye in return for having the ptosis improved.

With the eye more open, it will be more exposed to the wind, AC, dust etc.  These can all dry out the eye more. Immediately after a ptosis repair, most people will have some irritation.  This is due to the blink being incomplete due to the swelling, and often there is inflammation in the glands that make up the tear film, so it evaporates more rapidly. Artificial tears usually are sufficient to help with  this.  I have found that there is usually less dryness with Mullerectomy than with an external ptosis repair. As long as you don't have severe dryness you should do fine.   

The purpose of ptosis repair surgery is to restore normal function to the muscle that raises the upper lid. Patients with ptosis have greater corneal coverage than a normal eye (the lid crosses the colored part of the eye and often the pupil). Following ptosis repair the lid is normally positioned again at the top of the colored part of the eye, exposing more the eye to the air. Some patients with low tear production, "dry eye" may become symptomatic and require additional lubrication (artificial tears) at least temporarily. The eye will usually adapt with time and the need for drops will diminish. Patients with severe dry eye may not be good candidates for ptosis repair until their underlying problem can be corrected.

In any patient, when the eyelids are drooping, this limits how much air, wind, and light gets into the eye.  When the eyelids are elevated surgically in a ptosis repair, regardless of the technique, your eyes will be open more and thus more exposed to the air, wind, etc.  In some patients, they experience more dryness.  Most patients tolerate this well, with some increased use of artificial tears.  In some patients with severe preoperative dryness, this may be quite an issue.  One option is to try taping your eyes up in an elevated position at home for a little while and see what you think.  This should give you some idea as to how you will feel after surgery.  You can also discuss with your surgeon to minimize how much the eyelid is elevated.  There are a lot of options.  In worse case situations, sometimes the surgery has to be reversed if the patient is intolerant.   Good luck!