Why is the landing gear lever not set to OFF on the ground?
Because there's no advantage to doing so (since the pumps will soon be off anyway, as the question notes), and this would create the risk that the pilot may overshoot the Off position to the Up position, which could, on a bad day, lead to an inadvertent gear retraction and major damage. The over-center locks are good, but do you really want to bet big bucks that the combination of 3,000 psi + a bad bump in the pavement will never produce an illustration of Murphy's Law?
Okay, there is one other safety to prevent overshooting, but that solenoid can fail, and if it does, watch out! There is simply zero need for this.
Also, depending on the plumbing, you might remove hydraulic pressure from the nosewheel steering by doing this (though I'd have to check the diagram to be sure).