Minecraft itself perhaps does not care, but a 32-bit JVM process will have a 4Gb heap tops (3Gb on Windows) that will make the playing experience either unpleasant (i.e. constant garbage collection running in parallel and stopping Minecraft worlds) or will plainly fail as Minecraft creates a very large number of objects, esp. in large worlds, that won't fit into the JVM heap – each thing in Minecraft is a Java object.

Maybe it’s written in “pure Java” in the sense that the Minecraft source code is all Java. (Maybe. I don’t know.)

However, it definitely has dependencies that are not pure Java. I think Minecraft used to use LWJGL, I’m not sure if Minecraft does use it any more, but I wouldn’t be surprised if LWJGL or other libraries dropped 64-bit platform support for various reasons. Inside LWJGL there’s use of JNI.

Oracle stopped distributing JVM for 32-bit Windows after Java 8. Minecraft was using Java 17, and there were other 32-bit distributions for it, but I couldn't find a 32-bit distribution of Java 21.