Given the relative size of the places, and the connections available at them, split ticketing must indeed be a factor! I would be happy with a reliable 2 tph between Oxford and Birmingham with half going via Solihull, always my preference in the past as these services were both faster and quieter.
Split ticketing accounts for a lot, but the underlying fault lies with the pricing system that encourages split ticketing to exist in the first place.
Thing is, it could well be that train service planners believe the misleading figures that exist, and provide inappropriate services accordingly

Actually, thinking about this, perhaps this could at least partly explain the modern obsession with stopping trains as frequently as they do ...