Why was India never given a seat on the security council even though it was equal to China in every way until the 70s when China began its rapid economic rise?

If you are referring to the UNSC Permanent Members, the UNSC was formed in 1946 and India was not a sovereign nation until 1947.

As a result, and since the UNSC has never gotten any "new" permanent members (only had representation change, e.g. Russia-USSR, PRC-Taiwan), this has never come up.

I will not speculate why there have never been any new permanent members (why that process has never been initiated), only that there have not.