It is a contract but not necessarily the contract. The proposed written contract may have more terms and conditions. By buying and selling without a signed contract, you're agreeing to basic price and quantity without having to read and agree to a potential litany of other terms. Like what happens when you buy something from a store.

So it may not be relevant for this case, but in general buying and selling without a contract saves you from having to pore through legal details.

And some of those contract terms can be more or less favorable to one party than what a could would decide in the absence of an explicit contract stating otherwise.

So basically they prefer "signing" to contracts that are poorly specified and where any disagreement (significantly moreso than a well specified contract) results in a lengthy and expensive legal process for both parties. Which is fine as long as they know what they're getting into.

Yes, but the specific terms and conditions are not specified and you have to fall back on general law instead of contract enforcement.