OP can't mine testnet bitcoin, because it costs a lot. Correct.
OP desires testnet bitcoin. Also correct.

It'd be reasonable to assume that testnet coins do have a market value, because there's demand and there's supply. However, this should not be the case in here. The moment the community starts treating tBTC as a product or money, it'll essentially nullify the demand, because their only purpose is to help on testing. They exist only to help us improve and understand the main network's benefits.

Therefore, the owners are incentivized to give them for free.