Are all tires on an aircraft replaced at the same time?
Nope.. they are replaced 'on condition'. Exception is when one tyre has deflated before landing (cockpit indication and later confirmed) then the good tyre on the same axle has to be replaced irrespective of condition.

Tyres have a hard life and get damaged by rubbish on the runway and stand. Also airplanes don't always hit the ground straight .. when they come in at an angle  it wears the side of the thread more on that tyre.