It's not that GPUs don't support fp64, it's that for domestic gamer GPUs fp64 arithmetic is normally ~1:32 performance of fp32 arithmetic.

e.g. 1080gtx

    FP16 (half) performance
    138.6 GFLOPS (1:64)
    FP32 (float) performance
    8.873 TFLOPS
    FP64 (double) performance
    277.3 GFLOPS (1:32)

e.g. 3090rtx

    FP16 (half) performance
    35.58 TFLOPS (1:1)
    FP32 (float) performance
    35.58 TFLOPS
    FP64 (double) performance
    556.0 GFLOPS (1:64)

Only generally 'tesla' class cards targeted at super computers have a 1:2 ratio (e.g. v100, A100, Titan V). Note, I believe Titan V is the only Titan series GPU with good double performance, as the Volta architecture was never available to Geforce GPUs.

https://www.techpowerup.com/gpu-specs/geforce-gtx-1080.c2839

https://www.techpowerup.com/gpu-specs/geforce-rtx-3090.c3622

https://www.techpowerup.com/gpu-specs/tesla-v100-sxm3-32-gb....

https://www.techpowerup.com/gpu-specs/a100-sxm4-80-gb.c3746

https://www.techpowerup.com/gpu-specs/titan-v.c3051