https://www.pcgamer.com/sega-explains-why-shenmue-1-and-2-run-at-30hz-on-pc/

When the Shenmue re-releases were announced for PC earlier this year, Sega tentatively promised "modernized features" without naming specifics. Now, the publisher has revealed more on its new settings, its reworked control scheme, and why both games cannot run above 30Hz on PC. 

In an FAQ posted to Shenmue's corner of the Sega website, the publisher explains that "throughout the engine, game code and scripts there are hard coded assumptions about a fixed 30Hz frame update rate". All of which means the games "need" to run at 30Hz.