What is the excuse of govt. to keep FSI of indian cities so low ? Even with this low FSI,our urban infrastructure is so pathetic.

Never try to find logic in what govt does.

You are thinking too much here

FSI is low because our infra is pathetic. If they increase it then our infra would become even worse.
The water supply/ waste disposal and electricity network is already stressed. It will implode if FSI is increased.

And Singapore/NY have a high FSI because they have limited space so they can expand vertically.

Our cities are never built for future. With delay going into years. They would love to increase FSI, there is just no space for road or sanitation.