What causes ageism to exist to this day?

The reason is we have never dealt with it the way we should. In my opinion, the government is happier to deal with a tax audit of a company instead of human rights violations at the workplace. If they wanted to solve the issue, they’d already taken severe action against it. I’m aware that there are numerous laws and regulations to reduce it in society. Still, to my knowledge and experience, they didn’t do much about forcing companies and organizations to comply with those laws.
On the other hand, from a cultural perspective, it doesn’t matter how much you teach your people to be thoughtful about seniors’ rights because non-cultural factors such as the economy are still prolonging the existence of age discrimination. It’s a no-brainer that hiring the younger workforce is much more favourable and cost-friendlier than employing older workers. Private investors and the government still control the schools, so whoever has the money to fund them has more influence. We all know it exists, and I don’t think it will go away any time soon, at least the way we know it.