And the secret service has the luxury of being able to shut down whole blocks/towns when they think they need to, that’s not something a random bodyguard of a CEO can do every time the CEO drives around.
They do discourage "casual" attempts pretty well, and raising the difficulty constrains and pressures even the dedicated who succeed at striking (if not at achieving their ends) in ways that surely matter, but I think most of that has more to do with the shutting-down-whole-blocks and cordoning-off-entire-areas stuff. The strictly body-guard activity they do mostly just prevents sustained attempts—which isn't nothing, but CEOs aren't gonna keep those first couple bullets at bay with bodyguards. Broader behavior modification? Now that might work.