Founder of Pramp.com here. Coding and interviewing are orthogonal skills. Being good at the former doesn't mean you're good at the latter. My recommendation to you is to challenge your own assumptions about your interviewing skills. Do you know for sure that you can do a credible job at system design? Is the process you use when tackling a problem aligns with interviewers' expectations? How are your communication skills? The point is that practicing coding interviews with other engineers will allow you to get unbiased feedback about many of the aspects above.
As other commenters alluded, interviewing and coding are indeed two different skills. However, contrary to some of the comments, I don't think there is anything wrong with preparing for an interview or even taking an entire course for that matter. If it helps you shine and showcase your real skills to an employer then why not? I’ve seen many brilliant software engineers get rejected for positions cut out for them only because they were so nervous at interviews that they had trouble even understanding simple questions until right after the interview ended. I know that because I'm a co-founder at Pramp.com, a free p2p mock interviewing platform for software engineers. It's incredible to see the improvement in one's confidence even after a single practice interview.
Job search sucks for many reasons and for the most part it's the hiring companies to blame for. However, this is a given situation which will not change any time soon. Instead of ranting, candidates should put some time in researching what the companies they're applying to are looking for and act accordingly - e.g. personalize their resume to match job descriptions, anticipate interview questions and do mock interviews. Some resources that can help in the process: