Pulling the wheels for a brake job and finding one pad much thinner than its partner is a useful diagnostic. Uneven pad wear has specific causes. Here is why one brake pad wears more than the other.
Inner pad more worn
If the inner pad is much thinner, the piston is not retracting - it stays pressed on the inner pad. That points to a sticking piston or a collapsed brake hose holding pressure; see why calipers stick.
Outer pad more worn
If the outer pad is more worn, the caliper is not sliding - a seized guide pin keeps it from pulling the outer pad off the rotor evenly; see caliper slide pins. The caliper cannot center itself.
Side-to-side difference
One wheel's pads far more worn than the other side means that caliper is dragging - it brakes harder and hotter; see sticking caliper and wear patterns. That corner runs hot and may pull.
What to do
Service or replace the sticking caliper and guide pins, or the collapsed hose, then fit fresh pads and service the rotor on that corner; see install tips. Fit quality pads and hardware.