I can ask you the same question otherwise. Stick to the topic and answer if you know about what is being asked.
1) Why is options greeks data not available readily on NSE website
Because NSE website is not a research or analysis portal. It provides Trading Data like OHLC, volumes, OI, live and excellent eod reports.
Options greeks are not trading data. Basically It is no purpose for Stock exchange to calculate greeks. Hope you understand the point.
2) NEST terminal? Are they purposely doing it to make it hard for retail investors?
Not at all. There is nothing to hide in this. You must learn that Options Greeks are not trading data. They are kind of correlations between Underlying price: options price ( Delta ), options price change: change is underlying's volt ( vega) like wise for other greeks also.
So if the trading platform/website has the feature built in to make these greeks calculation they can publish that data. I think traders cockpit gives this greeks calculation, Kotaks trading platform has a advanced derivatives addon to its platform which even gives live changes in these options greeks. There are few more applications giving this geeks data.