Young folks learn rote (they just spent a whole bunch of years at school, doing exactly that), and adapt to it quickly, even if they don't fully understand the context or goals, so they are more likely to follow direction and process; especially if you can gamify the process.
Me, I like to know why, and to "play the tape through to the end." I'm also apt to say stuff like "Are you sure that you want to do that? You know it's likely to give you a race condition, if you use a different device. Might be worth it to consider a different approach.", and whatnot.
That kind of pisses in the punch bowl.
I actually hope one of the outcomes of post-pandemic normalization of remote work is less tacit age discrimination from companies that used to greatly prefer people willing to relocate.
Software building has always been for a purpose, whether scientific, engineering, publishing, trading. The value creators need tools and software is that tool.
Whether individuals are rewarded in the same way or the same amount is not because of FAANG. While large, they are by no means the majority of the economy.
Everyone needs software now. This pandemic shows the value of that, as well as of the research it empowered, the education it enabled, the community it connected.
It needs to be updated every few months to feel fresh, and the time horizon is very short. There needs to be analytics/engineering-work to enable this, but the goal itself is to adapt the user experience such that it maximizes adoption and screen time. What matters is fast iteration, creativity, and "fast plumbing".
The perception is that young people are more enthusiastic about this sort of work. More experienced people would be unnecessarily concerned with code quality, stability, technological trajectories, and so on.
This is a valid argument and true also in my experience.
The counter argument is that "software building" is to media companies (faang) what a press is to an advertising company. Problem-solving is not the relevant value-creating activity. The perceived need is for "creators" and "operators" for the "machine", rather than engineers and problem-solvers.
Screens today are spaces that need to be filled with "cultural content". The technology itself is a public good, and they want willing bodies that don't ask questions to turn the wheels of the "machine".
This is the rationale for ageism, whether valid or not.