What I think is missing is that if one buys a dedicated launch from e.g. Rocketlab, one is paying \$6m regardless of how much your satellite weights. If it is 300kg and can fit into their fairing you'll only pay \$20k/kg, if it weighs 60 kg you'll still be paying \$6m which works out as \$100k/kg.
If you're buying a whole rocket you're almost never going to max out the rated payload mass limit, so the whole rocket prices you're quoting for Rocket Lab and Virgin Orbit both look much lower in $/kg than what an actual customer will get.
Also the prices you're quoting from the rocket builders are just base prices for the rocket (or an ESPA port from SpaceX); but if you're building a cubesat you'll need to add additional hardware to mount it to the rocket and dispense it once you've reached orbit.
In contrast if you're buying from Momentus, Spaceflight, or some other rideshare aggregator the advertised price/kg is much closer to the final price you'll pay.
As a result if you're flying a larger small sat, a dedicated small rocket launch or an ESPA port from SpaceX will end up being the cheapest option. But if you've got something like a 10kg cubesat, Momentus will charge around $700k while the other providers nominally cheaper launches will cost a million and up.
Rocketlabs does do what I think are self-organized rideshares for customers going to common orbits. SpaceX doesn't sell anything smaller than an ESPA port for $1m; potential smaller customers for their rideshares either have to buy way more capacity than they need or go with an aggregator like Momentus who'll charge a higher per kg price to cover their costs but will sell smaller payload sizes.