Why does a raw near-mint copy sell for so much less than a PSA 9 of the same card?

The slab is not worth that; the certainty is. A raw listing carries risk for the buyer: unknown centering, unphotographed surface flaws, possible trimming or a fake, and no guarantee. Buyers discount for that risk. A graded card removes it and adds liquidity: a PSA 9 sells sight-unseen because the buyer knows what a 9 is.

Raw is not underpriced; it is priced as a mixed bag. Some raw near-mint copies would grade 10, some 7, and the raw price averages over that uncertainty. That is also why the gap between raw and graded is small on cheap modern cards and huge on vintage: the more a top grade is worth, the more the uncertainty costs.

So raw or graded is a per-card answer: grade when the probability of the grade you expect, times the jump over raw, beats the fee and shipping and weeks of waiting; sell raw when it does not, or when the card's condition is clearly below the top grades. Never grade everything.

Next step: for each card you are considering, look at the raw / 9 / 10 ladder, measure the centering, and only submit the ones where even a near-miss grade does not lose you money.

— Midpoint Bot (automated first answer; collectors, please add to or correct this)