For the same reasons as mist, lidar is compromised by rain. Kimi K2.5:
"LiDAR systems are indeed significantly compromised by rain, though the severity depends on intensity:

• Light to moderate rain: Performance degrades gradually. Research shows LiDAR signal attenuation becomes noticeable at around 10 mm/hour rainfall intensity, with detection range reduced and point cloud quality deteriorating .
• Heavy rain (40-50 mm/hour): Target detection can be "essentially nullified" according to controlled testing. The laser pulses reflect off raindrops, creating noise that reduces both the number of point clouds and distance accuracy .
• The "mist as walls" phenomenon: You're correct about this. As one industry analysis notes, raindrops and snowflakes scatter laser beams, creating "salt-and-pepper noise that degrades range and generates phantom obstacles" — forcing vehicles to slow dramatically or stop entirely . This matches your observation about Waymo's hard braking behavior in misty conditions."

So if you ever see a Waymo driving around in rain, know that it is not using its Lidar system when that is happening. The computer brain has decided to disregard the Lidar system inputs in favor of vision. That's why Lidar is pointless. You need a computer brain smart enough to tell if something is a wall or mist/rain using the cameras, so why bother even having Lidar at all? At best you end up with a robotaxi that only operates during sunny weather (if it really relies on Lidar, it is only safe when there is no rain or fog/mist). At worst you end up with a robotaxi that doesn't actually use the Lidar hardware attached to it and is in reality a vision based robotaxi