Because the way the code is compiled or hand optimized even, which kind of extensions are used for ARM vs x86 (SSE, AVX and so on). Many of the worloads used in Geekbench are straight up directly accelerated, which is fine if the only thing that's requiring CPU power on your machine is Javascript, but not so otherwise.
Other synthetic benchmarks of memory bandwidth and so on use all acceleration features of the plafortm on ARM but don't support AVX2 or AVX512, although some other workloads in the benchmark do. And of course you would have to choose exactly which instruction is used in which scenario in which processor(Intel vs Zen 1 vs Zen 2) in order to have the same kind of optimization as for ARM processors. Then comes the issue that vector operations are "hand-tuned", which is not realistic and depending on the skill of the programmer and their affinity with a given uArch can yield vastly different results. Which is why they should either use the fastest library for each processor, or leave all the optimization to the compiler.
The only way to do a proper comparison between two uArch is with an open source benchmark compiled specifically for the processor.
