I’ve seen the same thing, although they’ve not been on the market for very long. I think some realtors are keeping them off the MLS so they can get both sides of the commission.
No matter what they are saying in the news, the market is still pretty strong, and if a house is priced right, it will sell within a week. If the seller is “reasonable” about pricing, the agent has very little incentive to put it on the MLS, IMHO.
If I were a seller, I’d light into my realtor like nobody’s business if they did this to my listing, though.