
Why are guys with good packages and good looks and upper caste are not able to find girls on matrimonial apps? What are they looking for?

How upper caste has any significance here? Regardless of your economic status people are going to look within their caste only it's not like UC guys have better market in marriage compared to OBC or SC guys since market is within your caste only.
Also basically rich and good looking guys usually demand high dowry or other high standards, while women demand high income and nuclear families. Unrealistic expectations. Once you power your expectations Marriages are easy.

Aren’t intercaste marriages happening in India now?

Same reason why well educated mature and good looking women aren't able to find a husband.
Both sides have high standards, unwilling to compromise. They want perfection when it doesn't even exist.

Own house in metro cities
Vibes matching