Why do pre-op do not have full sex reassignment?

Pre-op means before the surgery, and implies that there will be surgery. That is not the same as a "non-op" transsexual, which is often a misnomer, since a person can't both hate their genitals so much to want them removed and also desire to keep them. Transgender would be a better term for them, though every person has a right to call themselves whatever they want.

Anyway, if a person is of the type that will get surgery, then it could mean that they don't have the money, cannot find the medical support needed, hasn't been approved for surgery, or has been disqualified from surgery due to health, age, weight, STDs or other concerns.

As for why non-ops don't get the surgery, it is often because they don't have transsexualism. They are transgender, but don't have transsexualism.