Barrels get reused because not all the oak is necessarily given up the first filling. Even if neutral it still provides micro-oxygenation. Everything the chips have to offer are used the first time. Staves I guess could potentially be sanded and re-toasted.
I tried re-using the 6 oz of oak cubes I added to a 54 liter / 14.25 US gallon barrel for 3 months. The recommendation was 2 oz cubes per 5 gallons; hence the 6 oz.
After that, all 6 oz went into one 5 gallon carboy for 3 months. The wine absorbed some character, but it was not a lot. IME cubes are used up after a single use as well.
As Fred stated, barrels continue to have value as a container long after the oak character is used up. I purchased a 10 yo barrel specifically because it was neutral -- I add the oak products I want (American, French, Hungarian, mix-n-match) so I can achieve the character I want in the same barrel.