Well there’s the simple reason, and there’s the historical justification for it.
The simple reason is: BECAUSE THOSE ARE THE RULES.
And if it were up to me, you’re not putting enough accents on Greek words. The blanket rule that all monosyllabic words are unstressed, whether they are function words or content words, does not correspond to how the language is actually spoken. And it was the kind of rule only a committee could come up with; it wasn’t the monotonic system that actual linguists like Kriaras had come up with.
Ancient Greek had pitch accent, and did not write any of its accents down: it was tolerant of accentual ambiguity, whether in location or kind of accent. In fact, Ancient Greek was also not written with any spaces between words. Writing and reading were difficult, and not widely known skills.
When the pitch accents started shifting to stress accents, scholars started indicating the different kinds of accent and their location on words, so that Ancient poetry could be read properly. That gave rise to the polytonic accentual system (“many accents”), which was the only way to write Greek anywhere for the next two millennia.
Starting in the 19th century, there were spelling reform proposals to simplify the accentual system, which was recording pitch and breathing distinctions that had died out two millennia ago. There were occasionally atonic proposals (“no accents”), to do away with any indication of stress. But the majority of proposals were monotonic (“single accent”), and in 1982 that was the reform that prevailed, after being de facto in wide use for a decade.
Why monotonic and not atonic? Greeks will tell you that there are still words that differ according to where you place the accent; and that is true. But comic strips are written in all caps, and so are headlines; and Greeks have no problem reading them, with only very occasional use of accents as disambiguators (almost always Ή “or” vs Η “the”). You’ll also often see no accents used in texting.
The real reason is that completely removing accents was a step too far, for people who had been writing and read accents on every word all their lives.