Take a look at how many threads are running on an idle Windows system. It approaches 1,000 (or can even exceed it with certain configurations).
Some of those threads have been dedicated to making single-threaded applications run faster, but a good 20% of them will be triggering or handling interrupts and user and application events while any decently complex program is running.
Super old games whose game logic, audio code, and graphics code are in the same thread likely already run well enough on a Sandy Bridge CPU at stock clocks that they are meaningless for this discussion. They will run even better on Ryzen, and more cores will help offload other tasks which may be running in the background. Disable a few cores and play those games and you'll possibly see it first hand.
One such game, that I play to this day, is of Age of Empires II. It is horribly limited by processing power on a single thread, no doubt. I used to run it on my old Core 2 Duo E7200. Overclocking had no effect. I upgraded to a Phenom II X3 (2.8Ghz) and had a minor improvement. I overclocked to 3.2Ghz... little better.
I upgraded to an FX-8350, it ran just the same. I upgraded to an i5-2500k @ 5Ghz, it ran no better. I moved to an i7-2600k... still no better. I don't think Ryzen, no matter how fast it is, will help with it, either. It's constrained by its own internal design and will not scale.
Even the new 'HD' design has such a behavior. It has now split the game logic and graphics pipelines, which helps smoothness when things are going bad, but the game still becomes unplayable with too much action.
You just can't fix those types of games with more CPU performance. They are constrained by timings related to game logic or accessing and directing audio and graphics pipelines. Those timings are dictated by system buses, storage, and other factors that don't respond to IPC improvements (they have max ILP already) or particularly well to CPU frequency. They just suck and will suck until whatever is bottle-necking it is resolved... then there will be another bottleneck.