Even if one percent of them were psykers- which is a massive over-estimate- then only a tiny fraction of that fraction would be Alpha level. And the Galaxy is not unified- much of it fights itself.
And the powers of such people are defined by the system, and what they can do is hurt people, many people, but they have not the power to win battles single handed (because they are designed for a game system, so that would be dumb; they are costed according to their power, and there are many things more powerful) so even this tiny tiny elite, most of whom would be executed on sight, do not have anything close to planet-destroying power.
It's just conceptually against how the 40K universe works. Thew most powerful psyker of all is the Emperor, and what did he do in the prime of his life? He fought people man to man, because no psyker has the power to wreak such remote havoc. His final fight with Horus was more like a Lightsabre duel than a planet-destroying battle.