Because AI exists in the universe with outside forces and has the same risks as anything else for dissolution of its complex state ("life," if we are being generous): run out of energy (starve), extinction (Earth blows up) or dissassembly (i.e. a virus).

The biggest weakness of AI as a self-sustaining life form is the lack of resilience to something like a virus. Having something live forever is easy - reproduction and evolution are an answer to provide diversity and robustness to viruses. Like if COVID was 100% deadly it wouldn't have wiped out humanity because of natural immunity in some % of the population.

So if AI is to persist it would at least need to co-opt evolution as a survival strategy, or come up with something better.