I meant to say a DC-DC doesn't share a lot of components with an inverter, I should have been clearer.
But also note there are a couple different design types for PV and DC-DC controllers. For example PWM PV controllers are pretty simple, just a bank of FETs typically. The MPPT controllers are more complicated, typically converting DC to AC and back to DC. Most MPPT designs need an input voltage substantially higher than the output. DC-DC chargers for alternator charging are often required to output a higher voltage than the input. Its tough to optimize such a design, because solar panels are current limited, but have a max power point below their open circuit voltage. While an alternator is not current limited (at the rate most DCDCs operate at). So there is no need for complex logic. In addition the switching speed and behavior of the input side of a solar charger isn't always ideal for an alternator, where the loading can cause odd behavior or electrical noise.
The main reason you don't see all-in-ones as often, is due to replacement and repair costs. Lets say you upgrade your solar array, or change its voltage. Does that mean you need to replace your all-in-one ($$$$)? What about about failure, for marine applications, failure tolerance is reduced when a single point of failure exists for all these critical systems.
The other reason in my view is all-in-ones are okay at a lot of things, but rarely perfect at them. Being able to select a solar controller and DC-DC charger specifically for your usage case presents more configurability.
That doesn't mean that I wouldn't use an all-in-one if the specs and pricing were good. Especially if it reduced end user configuration, and had a convenient control/programming interface. Victron with there easy end user firmware upgrades is a great example.