The short answer is: supply and demand.
SIA is not exactly a ground breaker in cadet recruitment. Why still no female cadets for SIA (Silkair yes)? So why should these people in charge stick their necks out just because of what you (a mere cadet candidate and not some world renowned industry expert on LASIK) say?
Until such time that they become so short of applicants (and then start relooking at applicants that were OK except for the eyesight component), they will just take the easy path and select the most qualified.
The long answer is: attitudes.
Until today the requirements / preference is that pilots wear spectacles as opposed to contact lenses. How long have contact lenses been around? Compare that with LASIK - it is still an evolving technology and not even half as old as contact lens technology.
If the authorities are STILL reluctant to embrace contact lens technology, do you think they will just give a blanket approval to LASIK? Contrast that with glasses - wear glasses? No problem lah, just wear them when you work and have a spare pair handy. Can you see the difference in attitudes?
And of course, CAAS bureaucrats being what they are, will not risk standing up and proclaiming: Hear Ye! Hear Ye! LASIK is now 100% approved, same legal standing as glasses.
So, you want to take up this crusade, or just move on?