You can't compare a service like Robinhood (or Coinbase, the like) with a product like Google or Facebook:
1. Unlike these free services, at an exchange, you really are a customer. The business model is that you pay fees for transactions. That's not the same as a free "take it or leave it" situation. As a paying customer, it is far more reasonable and expected to get support.
2. The nature of the support queries are far more serious. This isn't about your Instagram app glitching, this is about money. People unable to access it, transactions not coming through, withdrawal errors, fiat/banking errors. All very serious matters where not only you expect support, you expect urgent support and typically human support.
Now combine the above issues (true support needed, and much of it human support), we add the third and fatal ingredient: user growth. These services grow by at least a few million users per month.
I don't know how many require support, but even a small percentage means you're on a constant hiring spree.
Google and Facebook scale up by just not giving any support. How do you scale up this fast giving real support, so not just an FAQ or chat bot?