I’m sure a crush washer could be replaced, but have you ever tried to find one? I haven’t, like you I just replace the plug.
Being curious, I actually searched around a bit, and found virtually no one saying it was a problem to reuse a crush washer. I didn’t find anything very definitive though (such as what a manufacturer has to say about it.
Honestly, I don’t think it is an issue to reuse a crush washer. The issue is HOW to reuse it. Most FSMs (factory service manual) have a torque spec for the spark plugs and that is ok as long as the threads are good and clean. If you add a little anti-seize, the clamping force on the crush washer will actually be a little higher than design, about 10% but that could be good as it will give the crush washer just a little more crush to insure a good seal.
Crush washers have a design compression to give maximum sealing, so most plug manufacturers just give a torque angle spec instead of a torque spec. This is based on the amount of crush times the thread pitch. So if the plug mfg says 7/8th turn after contact, then 87.5% of the thread pitch will be the ideal crush for the washer. Once it crushes, it does not rebound much like a rubber seal would. There is some rebound from the modulus of elasticity of the metal in the washer, maybe 1-3% based on the metal.
So basically, you can’t reuse the plug mfg recommendation on a used crush washer. The vehicle mfg torque spec should be OK.
However, I still don’t like reusing a plug, especially on my Subaru which requires jacking up the engine to get to the plugs.
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I always went by the adage, if you take a plug out, you put a new plug in. Almost no car manufacturers recommend just inspect the plugs anymore. OTOH if you’re looking at the plugs for a running issue, inspect, correct any issue (eg. rich/lean) and still replace the plug.