But a lot of smaller companies don't need gargantuan satellites like that and can get a lot done with 100 kg. If you only need to put one or two satellites up it is not cost effective to spend lots of money on a large launch vehicle, so you buy space on another launch that has spare capacity and is launching at a compatible time/inclination. When you do that, you are not given priority and will have to wait if the #1 priority delays the launch for any reason. So Rocketlab can capture a market segment by letting small companies have their own launches.
It's kind of like getting the individual packets of ketchup from McDonald's vs the family size bottle of ketchup from the grocery store. Sure, the family size gets you more ketchup for your dollar, but you don't normally need that much.