I don't get why there's standstill traffic every time it rains in Makati/BGC

Make it make sense.

It's raining but cars are water-proof and can still run so there's no relationship between traffic and rain. There shouldn't be any traffic if there are no floods and there are no floods in Makati and BGC. So why is there always standstill traffic when it's raining hard? It's insane to me.