> I don't understand why Americans buy these kind of (expensive) processed and branded food products. Why not just buy pasta, the cheese (grate it if you want) and some other seasoning, as the raw individual ingredients?

Same reason we don't all run OpenWRT on our home routers - yes, it's objectively better and possibly cheaper but most of us don't have the time to prepare the meals ALL the time.

For us, the kids prefer it (even over homemade), and once a week we let them make it themselves for lunch. Done and done.