It's not; the US has launched more people into space; most of them using far more complex vehicles (reusable shuttles); some of them into far more dangerous missions - beyond the low earth orbit, to the Moon and back.
The US has had no fatal in-flight accidents involving simple rockets and space capsules comparable to Soyuz despite going much further and flying to the Moon. Their accidents happened with an experimental space plane (X-15 in 1967) and space shuttles (Challenger in 1986, Columbia in 2003). Russians spent over 20 years on their space shuttle, but it only flew a single unmanned test flight. The US space shuttle flew 133 successful manned missions out of 135 launches. Likewise, the US flew 199 missions with X-15 space plane, whereas its Soviet counterpart MiG-105 flew only 8.
In other words, Russians never got to flying at scale the kind of missions that killed US astronauts. You need to take that into account when comparing safety records.
> As of March 2024, in-flight accidents have killed 15 astronauts and 4 cosmonauts in five separate incidents.