Your car floor is wet when it rains due to a leak or deteriorating weatherstripping. The only way water gets into a closed car is if there is a hole somewhere or if the weatherstripping is bad.
Weatherstripping is the rubber that lines all your car’s glass. This rubber can wear down, peel off, or become a weak seal. All these can allow water to get into your car, leaving your floor wet.
You can try and fix this issue yourself or head to a mechanic to get your car waterproofed again.