Presumably you're being deliberately obtuse TabbyMumz - nobody said there were masses of kids with floppy necks, nor that anyone had "invented" putting babies on their stomachs. Giving something a name is simply a shorthand way to refer to pour promote doing something, it isn't a claim to having "invented" it, and pretending to think it is is just daft.
After the 1990s Back to Sleep Campaign (which also didn't "invent" putting babies to sleep on their backs, obviously, but just promoted it as safer) a lot of new parents were afraid to put babies on their stomachs at all any more, and research showed this leading to a rise in all sorts of minor problems, including flat backs of heads and poor muscle development and more babies being slow to reach first year milestones.
Although the babies usually catch up, there are studies suggesting delayed development in the first year can have a knock on impact later in life for some children, and there was no need for the development of those children to be delayed - "tummy time" was then coined as a handy name to promote to encourage parents to put babies down on their stomachs to play when awake and supervised (the same way "back to sleep" was coined in the 90s to encourage people to put babies in a safe sleeping position).
It took a few years for the trend towards flat sports on the back of the head and slower motor development in the first year to be noticed and 2 and 2 to be put together, hence the phase when back sleeping was advised but "tummy time" not mentioned.
Its properly weird to be so snide about a very simple initiative promoted by physiotherapists and paediatricians to help children get a decent start in life.
Most things have been done before since time immemorial and nothing is really "new" but that doesn't mean that people don't sometimes stop doing very practical things due to fashions or peer pressure or shiny consumer products offering tempting promises, or that research breakthroughs aren't sometimes made that do make new ways better (as in sleeping on the back and tummy time).