A client can choose to require a cover letter or not. It is still available on Upwork but you can come across a posting that doesn't ask you to submit a cover letter.
I also hire people without doing interviews.
Am I worried about hiring the wrong freelancer?
No, because if they are wrong for the job I can simply close the contract and get someone else.
It's not like I am having them relocate to my town to start a new job. I'm hiring them to create a logo or do some css design or something.
Interviewing them is not going to tell me much. Either they do good work or they don't.
I don't try to analyze a contractor's portfolio to determine if they stole the work from somebody else.
If I like their style and hire them and they don't produce the same quality of work for me, then I can close the contract.
If I use hourly contracts I don't have to worry about whether I give them a chance to finish or whatnot. I'm paying them for their time. If after 30 minutes or an hour or a day or a week, if I don't like what I am getting, I can close the contract. They have been paid for their time. We can both move on. I do not need to explain or justify why I am closing the contract.
In practice, I have not found it necessary to close contracts before work is finished, but I could if I need to, so the answer is no, I am not worried about hiring the "wrong" contractor.