Synth music style invented around 30 years ago on technology that didn’t exist a generation ago: "This sounds old-fashioned, the developers need to evolve."
Orchestral music played on instruments that have existed for thousands of years, in a style that’s existed for hundreds: “Finally, a modern sounding ost!” 
The same recording fidelity could be used to hire a rock band to do your soundtrack, but very few games do this, except maybe for an ending or a handful of cutscenes.
Meanwhile games that use synths usually emulate older video game hardware with a sharply limited number of tracks. Very rarely do you see someone (Disasterpeace) throw a dozen synths at a time at something.