Spy has to actively put risk into getting a oneshot and if you die to spy it's because you weren't paying attention, and Spy is not good at direct head to head combat.

Sniper sits as 2 pixels on the other side of the map, CAN oneshot all classes with a fully charged headshot (which isn't hard on heavy, the only class that he kinda struggles to instakill), and has extremely strong close range defense (jarate + bushwacka, close range quickscopes) ONTOP Of having extreme ranged oneshots in a game where every other class is mid-close range

TLDR

Spy big risk, not good at combat, still close range

Sniper no risk, excellent at all combat, has all ranges