> For example why do the vast majority of animals not live beyond 20 years, even when we try to keep them alive as long as possible (dogs)? If the environment was different would longevity be different for all species, e.g more oxygen in the atmosphere or a longer day-night cycle? Etc.

A lot of animals just don't need to live longer than 1, 2, or 20 years to reproduce. There is no evolutionary pressure to do so. From our human perceptive life is valuable on its own merit, but nature doesn't see it that way. All it really cares about is replicating those precious DNA molecules, and different species have different strategies for that. For some species, it takes a long time to produce offspring. Whales, for example, live a long time as well. They're huge and it takes a long time for them to become fully grown, find mates, produce offspring, etc. There is a lot of evolutionary pressure to live long. Humans live comparatively long because it takes a long time for individuals to acquire the skills we use as a survival strategy.

Not too much is known about the lives of Greenland sharks, but there are probably some evolutionary pressures for them to live long lives.