I do not understand why academics think all people on Earth were always at the same technological level at the same time.

They don't. Archaeologists are well aware that terms like "Hunter-Gatherer" and "lithics" subsume huge amounts of diversity covering hundreds of thousands of years, but they've stuck around for legacy reasons. Quoting the intro from my old textbook (The Lifeways of Hunter-Gatherers: The Foraging Spectrum):

    [...] in the models developed in the twentieth century to describe hunter-gatherers, variation was something to be explained away to uncover the essential hunter-gatherer.

There are thousands upon thousands of recognized "industries" of distinct toolmaking traditions and that's only a minuscule proportion of what must have existed.