That's actually what you CAN do, because futures are inherently a leveraged with built-in margin. You don't get a "margin" account in futures because you already abide by margin requirements with your cash.
I believe there are more exotic and complicated account setups for futures accounts at full-service brokers, such as using Treasuries as collateral, but I am not sure on those bits.
https://www.fool.com/the-ascent/buying-stocks/margin-account-vs-cash-account/
Quoted from the link:
"Futures trading requires the use of margin, so you typically can't trade futures in a cash account. Options trading is available, but it's more difficult to write options contracts. To write options, your positions would need to be covered with shares of the stock in question or enough cash to cover your obligations if the options are exercised. These situations are where the choice between a margin account vs. cash account can be very important."

