I've generally assumed the simplest explanation is that many of these weakly-supported procedures are regular, consistent income streams for the people who perform them in the US: my four wisdom teeth (that were causing me serious issues at age 19) cost $2k to remove nearly 20 years ago, and I know colonoscopies are billed to insurance in the thousands. There's not much incentive to move to cheaper tests or wait-and-see, when you can just do it to everyone who reaches a certain age by default.

Presumably flu shots are good business for the manufacturers, though I'm not sure about the science. After having the flu as a healthy late-twenty-something a while ago, which was...intensely horrible, I've chosen to get it ever since.

Not sure which country you're talking about, but in Germany, the public health insurances cheerfully pay for annual flu and COVID boosters for everyone.

Flu may not be too dangerous in people who aren't elderly but it still sucks. Can a non-elderly, not in any other high risk group, person get seasonal flu vaccination in Europe if they ask for it, and is it covered by European health care systems?