And when companies sample check a tremendous amount of produce in a greenhouse or a truck and decide the infestation prevalence is below miut matzuy, why don’t we say that 3 insects makes the whole thing muchzak?
Because it is clear from the מרש״ל and numerous other achronim that ג תולעים is a not a YEDIAH that there is another בריה in this batch or pot, it is merely a siman that this batch is different than the general product in the velt, and there is some סיבה that caused it to be muchzak bitolaim. In the words of the Chazon ish אבל אם נמצאו ג' הוחזק שכל הפירות באילן זה או בערוגה זו נלקו, וחשיב כרוב המצוי שהן מתולעות. In fact, if it is clear that the סיבה is מעלמא נפלו because the product is וודאי נקי we would disregard the ג. Similarly if it is clear that the ג shows us it is a miut hamatzui that is all we would be machzik it to be. Likewise, in a huge vat where ג conforms to known expectations of the miut sheeino matzui of this product, finding ג would change nothing. The מקור חיים in הלכות פסח says this even more clearly:
דלא אמרינן חזקת ג"פ כי אם בדאיכא דבר הגורם דמסתבר שיהיה כך, והיקש השכל מחייבהו שיהיה כך
However if we were to assur milk based on statistics as per your argument above, that is a whole different story. That means statistics are providing a ידיעה of what is here, and if that were the case, the YEDIAH that there is a בריה in this vat should assur the whole vat.