There are a finite number of shares - you can only buy what is available for sale. When you log into Robinhood and put in an order as a consumer you are doing

There are a finite number of shares - you can only buy what is available for sale. When you log into Robinhood and put in an order as a consumer you are doing it on a small enough scale that you don't see it but every "buy" request gets matched up with someone elses' "sell" request. When we are talking about a massive % of the total share count there simply isn't that many "sell" requests out there for you to take advantage of.