Part of it is probably tech-bro optimism, where we'll just magically figure out some technological solution to the problem, just like how we're totally going to solve climate change with fancy geoengineering.

There's also the fact that a lot of people are in an economically precarious enough position to not really have the time and opportunity to actually think too deeply about the future.

And finally - there have been various (alleged) historical practices of geronticide. Considering how society already treats the homeless, and how during Covid we pretty much did actual, not just alleged geronticide in all those nursing homes, it's not really a stretch that a lot of people would be willing to just... overlook things like that.