I think this is actually one of the more clever points Banks makes, although not explicitly.

Fundamentally, the Culture believes that living things (and their definition in this regard is remarkably broad) have a moral right to exist. Therefore, as a society they are not expansionist. In order to remain non-expansionist, the population must be kept stable and this has implications either in childbearing or lifespans. The average Culture human mothers about one child but that means they can't, on average live forever. Why they choose to have children at all perhaps also boils down to the future generation's moral right to exist, but also because they recognize that a renewing population means a renewing culture and Culture.

In this light, I believe it's easy to see immortality as a sort of childish self-aggrandizement comparable to wanting to become the ruler of some backwards planet. Skaffen-Amtiscaw (an artificial entity and citizen of the Culture) even remarks on Zakalwe's immortality as childish in Use of Weapons.

The Culture never appeals to nature – how could they, they are ruled by their Minds!

(Mind is a sort of very powerful artificial intelligence).

There is a lot of similarities between the Culture and Trek, they are both visions of post-scarcity humanity made impossible by the simple fact that humans could never be that nice.

If you knew anything about The Culture, you'd know it's not that simple.

Attitudes individual citizens have towards death are varied (and have varied throughout the Culture's history). While many, if not most, citizens make some use of backup technology, many others do not, preferring instead to risk death without the possibility of recovery (for example when engaging in extreme sports). These citizens are sometimes called "disposables", and are described in Look to Windward. Taking into account such accidents, voluntary euthanasia for emotional reasons, or choices like sublimation (abandoning physical reality), the average lifespan of humans is said in Excession to be around 350 to 400 years. Some citizens choose to forgo death altogether, although this is rarely done and is viewed as an eccentricity. Other options instead of death include conversion of an individual's consciousness into an AI, joining of a group mind (which can include biological and non-biological consciousnesses), or subliming (usually in association with a group mind).

-- https://en.wikipedia.org/wiki/The_Culture#Death

I want optional mortality, but am also comfortable with death, and I can imagine situations of survival where I would prefer death.