It’s not marketing, it’s development of new technologies, which require multiple iterations and in many cases the creation of other entirely new technologies and processes in order for something to be consumer-ready. Under screen cameras are not currently ready for mass consumer use - have you seen the terrible results on those devices that are trying to make it work currently? Apple can’t launch something like that without making millions of people angry. It’s going to take years of development before it’s ready for iPhone use, even if some brands are willing to put an inferior product on the market today.It's called marketing - they already have the tech for under-screen, etc but Apple will always milk it every year with tech i.e the notch gets smaller and smaller so people carry on buying each year that new thing! so then this new pill becomes the new thing for 2 years then the under-screen camera will be introduced..
And Apple is no different than any other product manufacturer, where annual updates drive sales. Car companies do it too, as do food brands, mattress manufacturers, sporting goods companies, clothing companies, etc. So in your mind, all these companies are holding back new and novel features every year, just so they can offer it up the next year, in some grand scheme to empty our wallets.