Bitstamp being overly nosy doesn't surprise me. They always took the cake for having the most invasive AML/KYC measures. I wouldn't put up with this kind of invasion of privacy. If you're in Europe, Kraken and Coinbase Pro are some other options that support SEPA funding.
Sorry, but why would your net worth be any of their business? My bank, my insurance provider, even the government don't know my net worth - why does a crypto exchange need to know? Just another exchange going over the top with ridiculous KYC requirements.
Net worth and annual income are pretty standard questions for verifying accredited investor status. When GDAX had margin trading, I remember using leverage was restricted to accredited investors. Doesn't really apply to Bitstamp, though...