Just because lidar is not working in certain places and certain conditions does not mean that it's always useless. It just means that an AV cannot rely [only ]on lidar. Lidar could be used as a suplementary sensor - that is providing additional sensory data when appropriate and switching to non-lidar mode when conditions require.
This is one of the things where AV can have a clear advantage over [even above ]average driver: a superior sensory system
If you switch to non-lidar mode for unfavorable condition, why don't just use the non-lidar for all conditions. A system that is safe for unfavorable conditions is likely to be the system that is safe for all conditions.
Because lidar allows faster travel in the modes where it works. When there is heavy rain/snow the only safe speed is very slow anyway. When roads are clear and dry I really want my car to go faster than the elderly walk (I walk faster than that normally), and lidar may be required for that mode.
